The Position And Shape Of The Continents Biology Essay


Volcanos are found near oceanic ridges and trenches where collided plates have caused subduction and spreading. Crustal friction is high near the surface at trenches, but also deep in the crust as the forces of subduction press plates into a pileup. Sea floor spreading forces plates apart at oceanic ridges, while new crust is formed from the upheaval of the mantle's magma. It is data like this from volcanos in addition to earthquake locations that helps corroborate the plate tectonic model.

The concept of Pangaea, one super continent 225 million years ago, has been suggested by geological and fossil evidence and that provided by the plate tectonic model. As Pangaea was torn apart, her fossils, animal and plant populations, and surviving geological features were cast with her remaining pieces. It's taken millions of years for the remains of Pangaea to come to rest where they are, to the plates and continents we now recognize as modern Earth. Continental drift is supported evidentially by plate tectonics; most of this evidence coming from mapping explorations in the 1960s. The mechanism causing the drift is the same that helps create volcanos: sea floor spreading. New crust is created where plates diverge or collide in oceanic ridge/rise systems. The system functions something like a conveyor belt for recycling crustal material: deep sea trenches are formed by colliding plates which subduct the old crust, melting it and whatever's laying on top of it, down into the asthenosphere. So, acting in harmony with these subducting crusts, shiny new crust is born: spiffy.

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Sometimes things get a little tricky when an oceanic crust meets a continental (the subductee), forming andesitic crust. This sort of crust can be found in island areas like the Aleutians and mountain ranges like the Andes. Where an oceanic ridge would normally form a deep sea trench, the diverging plates form (in these instances) into part of the ring of fire. Subduction is the blending and melting of the two types of crust down in the asthenosphere.

Where the Aleutian rely on subduction and the formation of andesitic crust for their volcanic and earthquake activity, Hawaii has a phenomenon of its own. It sits above a hot spot, or rather a volcanic area without an associated plate. The concept here is the plate is weaker or thinner than the area around it and magma always finds a way. Another possible reason behind these hot spots is that these areas are in fact hotter than the average and they sizzle and melt the areas around them. Either way, volcanos form as the Pacific plate moves northwest over this hot spot, creating the Hawaiian Islands. The evidence of the movement can be witnessed in the age of the islands, the oldest being the further northwest, Kauai (and the prettiest in my book) and the youngest being the island of Hawaii itself. There's soon to be a new baby in the family as seamount, Loihi grows beneath the water.

The life histories between unicellular and multicellular algae are extremely varied. Take, for example, the unicellular alga, Chlamydomonas, which has adapted to reproduce in an interesting way. Normal population growth occurs by haploid cell division, which is an asexual form of reproduction fostering a generation of genetically identical specimens. However, when times are tough, cells begin to fuse using syngamy therefore producing zygotes of a diploid nature. Of the fused cells, these zygotes keep the four undulipodia and if conditions worsen, the zygotes put up a cyst wall and head to the bottom. This is how Chlamydomonas survive the harshness of vernal pools. The zygote is dividing itself into for haploid cells or meiosis to get ready for the next rain when all is well with the world and our little Chlamydomonas are up and swimming again. Chlamydomonas well illustrate the benefits of being diploid: harsh conditions. They don't need a varied genetic supply to continue generation (as each diploid cell contains two sets of genetic information) and when conditions improve, switch back to a haploid state. Where a diploid must fuse cells (syngamy) to generate one new cell, a haploid is free to split and produce four new cells. This would be an advantage in the face of necessary evolutionary adaptions and could populate an area at a quicker rate than a diploid cell. The best of all is having the ability to switch between the two, like the Chlamydomonas, who in an extreme habitat like vernal pools; an organism must be well-adapted to survive.

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As an interesting bridge between unicellular and multicellular Algae, is Fucus or Brown Alga. Biochemically similar to diatoms (which are often unicellular), Fucus contains both chlorophyll a and c. Also found in this type of algae are chloroplast edges rimed with tiny fibers much like Chlamydomonas' 25-A DNA chloroplast fibers.

Multicellular algae, like Ulva or Green Alga, has a life history similar to that of all plants. Again meiosis is the process we're looking at here, but in this case diploid sporophytes are producing haploid spores which then germinate and, by the process of mitosis, grow, becoming haploid gametophytes. These haploid gametophytes beget (by mitosis) gametes and these little buggers form diploid zygotes by fusing together. After germination, mitosis causes growth and it begins again. You may start singing "The Circle of Life" at any time now.

Things get a wee bit more complicated when you look at Polysiphonia, or Red Alga, as it need more than just dinner and dancing to procreate. In this case, diploid tetrasporophytes are producing haploid tetraspores (through meiosis), which after germination and growth by mitosis, they become haploid gametophytes of either sex. Then, in total swamp creature fashion, one haploid spermatium nucleus meets up with a haploid egg nucleus and the resulting diploid zygote nucleus transfers to a supporting cell which gobs a mass of diploid filaments called the gonimoblast while the surrounding sterile cells gob into a covering known as the pericarp. Producing carpospores, this messy post-fertilization stage is called carposporophyte. Within a haploid pericarp, the carposporophyte contains diploid gonimoblast particles. Converted into carposporangia, the final cells of the gonimoblasts liberate a diploid carpospore. These carpospores cannot move, so they just plant themselves and germinate to begin this whole process again.

The alternation of haploid generations can be described by the production of gametes by haploid gametophytes though mitosis which fuse through syngamy and form zygotes. These zygotes grow up to be sporophytes. Spores and gametes are haploid, though gametes combine genes from separate parents, and must fuse to create the sporophyte generation. Similarly, the alternation of diploid generations can be described by the production of haploid spores by diploid sporophytes though meiosis, after germinating, grows into haploid gametophytes. It is important to note that gametes recreate the diploid condition by using syngamy, allowing the recombination of genes from separate parents.

The life history of a moss can be described by its sporophyte generation: in the beginning was a mama gametophyte spawned a sporophyte (from within the archegonium), the kid sporangium grows up on the seta with the protection of a split archegonium. The process of meiosis is creating spores inside these young sporangium and once mature, they nod and the calyptra fall and the spores are uncovered by a now open operculum. Back again, in the circle of things, around this opening of the sporangium lies peristomial filaments or rather, little teeth to aid in spore-wind dispersal and protection.

The life history of ferns can be described by its gametophyte generation: germinating into a sheet with the antheridia with the rhizoids and on the underside near the notch, the archegonia. The antheridia make a ton of spiral-shaped sperm which, when there's enough water, swim to gametophytes and their lovely archegonias. Once the sperm fertilize the egg, zygote is formed and immediately begins dividing to create the embryonic sporophytes (archegonium-based) and soil contact is established. Now that the sporophyte is off on its own, the mama gametophyte dies; so much for empty nest syndrome.

Moss has well adapted for life and reproduction on land because the sporophyte grows inside the archegonium and because of this, during early development, is nurtured both physically and nutritionally, by the female gametophyte. The sporangium is also elevated due to the elongated stalk of the sporophyte and the variety of spores produced is increased due to the recombination of genes before segregation while the spores are being produced because of the diploid genome. Lastly, those little peristomial filaments, or teeth help with the wind dispersal of resistant spores. Now on the gametophyte side, moss has well adapted for life and reproduction on land because of the insulation and reduced evaporation provided by the phyllids that cling to the stipe as they dry. The stipe is really just a cluster of filaments and because it never really touches the atmosphere, it can house internal food and water cells and reducing the relative evaporation rate because the surface-to-volume ratio has decreased. Increasing the transportation rate of food and water (elongating cells speed things up), cells are elongated inside the stripe. Specializing tissues creates a greater division of labor, i.e. reproductive organs (archegonia and antheridia), holdfast tissue, internal storage tissue in the stipe, phyllids, and external protective tissue. Specialized structures assist too; like splash cups help female archegonia get their dose of sperm.

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Ferns have not as successfully adapted for life and reproduction on land as moss because they require enough water to allow swimming sperm. They have adapted well in many other ways as they cast spores to the wind. The real success of ferns lay in its vascular tissue development. Highly specialized for the resourceful movement of food and water, vascular tissue is a conducting tissue. Where phloem primarily conducts food, xylem primarily conducts water. It is this ability that has allowed ferns to succeed, to reach permanent water by root, achieve amazing heights, and get an era named after them. Personally, I'm more impressed with moss.

Moss is limited by its gametophytes being only slightly terrestrial. Having no real vascular tissue, moss relies on cell exchanges for food and nutritional transportation. The high surface-to-volume ratio or the protonema is similar to those found in alga, and not well suited for terrestrial life. Conditions must be perfect (wet habitats or wet season) for protonemata to survive and produce gametophytes before they dry out and die. On the subject of lacking vascular tissue, this also means no leaves, roots or stems. Moss has phyllids for leaves, rhizoids for roots and stipes for stems. Without these tissues, compactness is the key, which is not in their favor in the case of rapid water loss due to their surface-to-volume ratio. This time the rhizoids dress in algae fashion; these phyllids are but one cell thick.

As I briefly touched on earlier, ferns face some serious limitation as far as life and especially reproduction on land are concerned. Ferns are so much like aquatic algae, spores can only succeed if landfall occurs in an area with enough water for sperm to swim in and its gametophyte, in nature is like a sheet of algae, thin, terribly delicate and prone to drying. These gametophytes have quite short lives, relative to the seasons. It seems that ferns require a habitat half-aquatic, as to not dry out and so their sperm can easily swim to new gametophytes, and half-terrestrial, as their spores must be carried by the wind and after the zygote is formed, soil is necessary to root the sporophyte. Perhaps this is why ferns were so successful in the Carboniferous, 30 million years ago, but less adapted to modern climates.

Compare and contrast the life histories, methods of reproduction and reproductive structures of conifers and flowering plants. How are these two groups adapted for life on land?

The life history of a flowering plant can be described after the pollination and fertilization process (which we'll delve into more in reproductive structures) when the flower begins to wither and its stamen and petals drop and fall. In a flowering plant, the seeds can be found in the locule and as they develop, the carpel elongates and its walls bulk up. The fruit dries upon reaching maturity and tension increases because of shrinking in this process in the capsule's outer walls. The fruit explodes once completely dry and matured; flinging seeds in all directions and rupturing the delicate areas between carpels. Seeds are dispersed by wind and wildlife hoping to find a new home where germination can occur and the process can begin again.

The life history of conifers can be described starting with the gametophyte at meiosis when the male cone scales or microsoprophylls bloom into microspores and the female cone's ovules develop megaspores. Microspores of pollen grains are produced at meiosis, which, along with a bladder filled with air, taken to the wind inside an ovule containing the megagametophyte. One functioning megaspore is produced in meiosis, and within that pollen grain includes a tube cell and a generative sperm cell. After penetrating the micropyle of the scale of a female cone, pollination occurs. Where the process is generally perennial in flowering plants, in conifers, this process takes a year for the tube cell to elongate from a pollen tube to reach the egg cell. Resulting in a brand new baby diploid zygote, the sperm cell moves through the pollen tube making its way to the egg to fertilize it. The embryo begins to develop then hibernates the winter, picking up the process when conditions are ripe. Spring returns, germination occurs and a diploid sporophyte has its day. When this sporophyte matures, diploid cones of both sexes develop and we see a new alternation of generation and another chance to sing from the Lion King.

The advantages of being a flower sometimes outweigh those of being a conifer (unless you consider long life) as the reproductive structures are flowers, allowing insects to do more of the work for them. Also, the ovules are hidden within the female sporophyllis rather than hanging out for everyone to see on the conifer. There are less gametophytes needed and produced with flowering plants and the process of double fertilization makes for an ultra-nutritious triploid endosperm. Lastly, the seeds in flowering plants that develop in the ovary or like structures are protected in fruits, which if you've ever had to drop an egg off a building in elementary school, you'd know is quite advantageous.

Flowering plants produce spores through meiosis inside the sporangium, and these spores divvy up to create male gametophytes. These little fellas only produce 4 cells (3 die) and when it's all grown up, it cracks to issue pollens grains (mature male gametophytes). This whole pollen grain has to be carted either by an animal or by wind to make it to a female gametophyte, and when he does, boy howdy, a pollen tube'll shoot down the pistil through the carpel to get to that lovely lady gametophyte. This carpel is home to the female sporangia and is actually the folded sporophyll. Much like her male counterparts, the female spores have germinated producing female gametophytes inside the sporangium. These ladies each produce one egg, and are subsequently fertilized and fused with the pollen tube and sperm to become a diploid zygote. Then the story picks up with the "Life History" section above.

Conifers produce pollen grains which, in the case of Pines, have a winged appearance at maturity and are somewhere around 50 micrometers large. This comes in handy as flight (instead of the primarily insect pollination found in flowering plants) is required for most conifers to reproduce (except in more alpine, bushy varieties where layering is observed). If the pollen grains penetrate the micropyle of a female cone, they're in business. It's a lengthy process, taking a year just to produce a zygote and then it has to wait for the right conditions to germinate. The most interesting part of the reproduction method of conifers lays in its seed/scale complex on the female cones, a feature which we'll explore in reproductive structures.

Flowering plants have a few unique reproductive characteristics. Picking up the story inside the pollen grain, two sperm nuclei are produced as a result of a second mitotic division, while in the ovule; the megasporocyte is developing, undergoing meiosis and resulting in four haploid megaspores. While three of these die off, the fighter remains, undergoing three more mitotic divisions producing a sac with an eight nucleate embryo (mature megagametophyte). Then it's pollination time and on the stigma, a grain of pollen germinates while the pollen tubes grow into the ovary, down the style, by way of the micropyle. Now, there are two sperm nuclei: one creates the diploid zygote and the other joins in to become part of the triploid endosperm. This is the double fertilization, unique to flowering plants. Then in the embryo sac, the wee one develops while the ovule helps to form that lovely armored seed coat, creating a fruit to protect it as it matures. After the seed is shed, flowering plants pick up cues from the conifers and hibernate until conditions are ripe for germination. Once the time is right, germination can begin and the embryo can grow into a flower producing diploid sporophyte just like its mama.

Conifers are blessed with a particularly interesting seed/scale complex in the female cones. These branches are fused (many times gloms of megasporophylls), a combination of ovuliferous scale, the umbo, and seeds. These branches begin as a mere leafy branch, reducing and fusing until the ovuliferous scale is created. On top of these ovuliferous scales are home to the young ovules and also the integument, which includes the megasporangium and the micropyle. In two years' time, the cone will grow, its ovules will be fertilized and its seed mature. Within the ovule lay the eggs, megagametophyte, archegonia, micropyle, and megasporangium. After a year of dormancy, the megasporangium is penetrated by pollen tubes. Outside the spore, reduced microgametophyte tube cells shed their pollen tube nomenclature. A story we've heard throughout these answers, but one embryo will survive. Surrounding this lucky embryo and haploid megagametophyte lays the megasporangium wall. Diploid in nature and produced by the female sporophyte, the whole structure is contained by a wall of diploid sporangium with internal tissues of haploid gametophyte and an embryo that is diploid sporophytic in nature (note the similarities in embryonic state in flowering plants). The whole shebang is what we call the seed (or eat as a pine nut). The megagametophyte nourishes the embryo as long as it's at home.

Flowering plants have successfully adapted to terrestrial life by specializing tissues for moving and absorbing water so they stay evenly hydrated throughout the plant. This is the idea behind extensive root systems as they support height, though roots cannot photosynthesize. The plants developed vascular tissues to move water around to let the roots do the holding on and soil absorbing instead of spending their days longing for sunshine. The water and nutrients (fertilizers) absorbed by the roots moves up the stem to the leaves and in return, the leaves send down shots of photosynthetic product.

Due to the conifers ancient roots, I'll begin to examine the adaptions the Gymnosperms have made to terrestrial life. When compared to non-seed bearing plants like ferns, gymnosperms have evolved a protective integument, which in turn, helped develop the ovule (integumented megasporangium). At the same time, the pollen tube evolved in a way that sheltered and placed sperm right next to the egg, all without the presence of standing water. Without the need for water to transport genetic material, they evolved past their aquatic ancestors into the terrestrial world. Conifers use the same vascular tissue system that all plants due, but in contrast, have adapted much further to withstand extreme climate conditions. Conifers (pines, to be exact), have taken this adaption even further by being drought-tolerant by reducing surfacing area, thickening cuticles, reducing size, thickening the epidermis and sinking the stomata. Conifers also use the power of the wind to disperse pollen, evolving the shape and design of the grain to take wings and soar. I'd say conifers have certainly come a long way. Typical of gymnosperms, pollination occurs when pollen grains find unprotected ovules and in most conifers, this action is carried out by wind. However, with cycads, pollen can also be distributed on the beetles present to harvest. This is understood by some botanists as an insight into flowering plants' animal pollination origins. Though most flowering plants are pollinated by birds or insect, some by wind and a few by bats, reptiles and mammals; it is insect pollination that is integral in both the success of and the coevolution of flowering plants and insects.

Apical meristems are tissues at work creating height and depth in perennial plants. The phloem is the highway that moves food from its sources where it's made or stored (like leaves or roots) to where the food needs to be (growing tissues or more storage). We know that that leaves photosynthesize, but they are also home to mesophyll cells where glucose is made. Glucose is packaged into sucrose and loaded into the sieve tube elements next to the mesophyll cells. Loading is a complicated process, with the osmotic concentration raising of living sieve tube elements. This is effectually lowering water pressure which creature turgor pressure due to xylem entering the sieve tube cells. The development of turgor pressure is possible due to the sieve tube cells' functional plasma membranes. This pressure moves the food and water to areas of less pressure or sinks. This is the process that allows leaves at the top of a plant to supply food for apical meristem growth, lower leaf pressure relief, food for root growth and storage. The strongest sinks on a plant are found in developing fruit, so much so that it can cause a halt of growth and other activities while fruiting or flowering.

Flowers are exacting, modified superstars of natural selection as they are extremely efficient specific pollination strategists. Unlike wind-pollinated gymnosperms, 65% of flowers are pollinated by insects (entomophily), with birds, bats and winds also used to fill the gap. Sometimes even lemurs, lizards, mice and opossums will pollinate a flower or two, so it's easy to see why flowers have adapted their colors, flowering times, nectar rewards, sizes, and shapes to match those best suited to get the job done. In fact, co-evolution of animal pollination and flowers has been going on since the Cretaceous, resulting in numerous cases of utter interdependence. It's not just the flowers themselves that have adapted; the fruits have efficiently found ways of dispersing seeds. You take a seed, cover it in a nutritious, fleshy and delicious fruit, and low and behold, your seed gets carried away, dispersed (and often fertilized). Some seeds take a less delicious route, opting for spines or burrs to stick to their animal pollinators, while some have wings for optimal wind dispersal. Some have adapted as sea-farers, like the coconut, to find some lonesome shore and take over in the name of Anthophyta. As insects are the pollinators for the majority of following plants, these plants offer odors and nectar rewards to entice visitors much like neon to a nightclub. As insects and plants evolved together, modern Earth can show the successful friendship everywhere you look.

As with conifer's adaption to terrestrial life, the origins and adaptions of cones begin with the cycad. To contrast modern conifer's seed/scale complex, the female cycad's cone are simple (as are the males). They are composed of ovule-bearing branches with modified leaves (megasporophylls). Modern female pine cones are much more complex, instead of branches of leaves; we find branches of branches as the ovuliferous scale is not a leaf (megasporophyll) at all, but a fused branch, flat and short.

The chaparral is a biome with a Mediterranean climate, dry, warm summer with common wildfires. These communities can be located around the Mediterranean Sea, but also here at home, along the west coast of the U.S. and South America, Africa and Australia. These areas may look similar but each house a group of unique organism, specially adapted to that environment. The most common type of plant in the chaparral is a woody and dense shrub, one well adapted to burning, and as these plants live in such extreme proximity, a healthy chaparral is almost impossible to penetrate, restricting amounts of light reaching the soil. Leaf litter also contributes to light restriction at the soil's surface, and very little decomposition occurs because the chaparral's plants do not provide a good environment for decomposing fungi and bacteria. As such, most plants in the chaparral form in stands, generally of the same age with few younger plants because of the duff covering and low light near soil.

Chaparral plants are well adapted to long dry summers because they are water conservationists and tend to germinate during their first summer, in areas of rockier soil, sending roots down between rocks where wet areas can be found even in drought. Some plants, like the coastal sagebrush, have adapted a dull gray foliage to reflect light, avoid water loss, and keep temperatures down, even on the hottest summer days. Another successful summer adaption is found with the greenbark ceanothus, which reduced leaf size in order to reduce water loss. A curious and sometimes dangerous adaption can be found in the waxy and highly flammable leaves of the Chamise, which evolved as a secreted thick waxy cuticle as a means of reducing water loss; it's a great fire-starter. Of those well adapted to mild winters, the gooseberry actually flowers in January, hosting a set of pollinators during a normally less-productive time. Also well-adapted is the manzanita is to both fire and mild winters, as much species of manzanita have seeds that are fire resistant, and some burl sprout; these shrubs also flower in the winter, fruiting soon after.

Climate is the key to determining the distributing of any biome, and it a product of: temperature, radiation, and available moisture. The climate require for the chaparral is located at 30-35 degrees latitude near subtropical high-pressure belts, north and south. Resulting in big subtropical deserts, these regions are laden with warm, dry air, while these belt's polar sides are temperate zones: wet, cool and polar air current-controlled. These are the meteorological conditions that cause the dry, hot, summers and wet, mild winters that define the climate Mediterranean and the chaparral. Around the world, these climates can be found in 5 places: the Mediterranean, central Chile, western California, western and southern Australia and southern South Africa.

There are four types of chaparral communities based on the height of shrubs present, general arrangement of fauna, and other plants present. Type one is of the Mediterranean nature and includes California's coastal scrub chaparral. These are sclerophyllous shrub communities found with perennials. The second type is the most common, the chamise shrub community, found in all chaparral areas worldwide (70% of the California chaparral). The third type is that of an open, low evergreen sclerophyllous forest, like the oaks of California and the Mediterranean (in Australia, type three is dominated by low lying species of eucalyptus). Type four lay at the limit of the chaparral; a complex mixed low woodland blend. The chaparral blends in succession throughout intermediate and higher elevations in California, though these are not part of the chaparral biome. Fire or logging may remove the natural conifer forest and mountain chaparral may replace it, but is a stage rather than the climax forest.

The influence of fire can be seen and felt throughout the chaparral. On a personal note, living in a successive mountain chaparral biome, one must be very aware of conditions, exit routes, topography, and property damage insurance rates during the fire season. In rural Calaveras County, it's the number one cause of claims. Often started by lightning, natural fires burn through the chaparral about every 30 years. These fires are quite beneficial, burning off the duff, releasing nutrients, and allowing light to reach the soil. It's easy to understand that the plants that survive the fire succeed, but some chaparral plants have evolved to need fire to reproduce. That's where the trouble with people comes in. Homes like my cabin and farm prevent fires from coming in and restoring the natural order of the chaparral. In fact, if a chaparral is left too long (60-100 years) it becomes senescent, too old for its own good; killing off plants because there is too much duff and no place to put it. These senescent chaparral are often what we find when people live in these areas and unfortunately, it's a deadly combination. When fire does strike, there is so much fuel laying around that it destroys, not only homes, but the natural abilities of the plants of rebirth after fire. Normally, it's only a few weeks before the comeback mechanisms of these plants become evident, usually after new moisture; they may never comeback if the fire has been too extreme.

While in senescent chaparral, these comeback abilities may be impaired, this function and adaptions like germination only after burning are a unique evolution to these fire-prone areas. As the chaparral is unfriendly to seedlings, with its low soil light and thick duff, this adaption, seen in manzanita and ceanothus, ensures that these seeds have the tools (and nutrients) they need to succeed. Wildflowers also take advantage of the burns, as they are not part of the mature chaparral, but require the increased soil light and nutrients to survive. Some species even require heat activation, and their presence at this stage in the recovery helps hold the soil while the chaparral grows back. This may take as little as four years.

To prevent senescent chaparral fire, federal and state agencies perform controlled burns to clear brush, create firebreaks, and attempt to control all aspects of the burn while it's on their terms. Though, I can personally attest to how the chaparral cannot be tamed, even by professional firefighters, these burns are very important as to prevent a massively destructive wildfire from which the natural wildlife cannot return.

The Bigpod Ceanothus has well adapted for water conservation by evolving its leaves to a smaller size so that less water will be lost because of surface area. This type of Ceanothus also adapted a light gray/green color which reflects light and reduces the loss of water. The top of these leaves are darker than the undersides, aiding in this reflection, and the leaves themselves are generally no longer than an inch, smooth-edged and oval shaped. Some relatives of Ceanothus (Cerastes) have taken these evolutions some steps further, adapting greater cuticle and leaf thickness, greater leaf mass, shallow root systems, leaves featuring encrypted stomata, and possess the capacity to uptake carbon at lower water potentials.