FRQ: Plant Adaptations to Land
Nina Yang, AP Biology B, Mitchell
Danzón No.2 - Arturo MÁRQUEZ (b. 1950) by mareah
Flowering plants have had to overcome many problems to adapt to life on land.
For example, the absence of an aquatic environment for reproduction.
Angiosperms consist of all flowering plants. Like gymnosperms, they have male and female sexual organs consisting of pollen for the male, and ovules for the female. Aquatic plants have their sperm spread by water, but angiosperms must rely on other means as they live on land. Some angiosperms are wind-pollinated. Their pollen is carried by the wind to other plants of the same species, thus cross-pollinating the trees and mixing their genes.
Other angiosperms attract pollinators using structures called flowers. Organisms such as insects or other animals can transfer pollen from one flower to the female sex organs of another. This also allows for cross pollination even with no water.
Their seeds, the plant's embryos, develop inside chambers, called ovaries, that are found in flowers and that eventually develop into fruits as they mature. When the ovules develop into seed after double fertilization, they are protected from environmental conditions and the like by the ovaries until pollination of the eggs triggers the ovaries to start growing. When they become ripe and mature fruits, they tend to be typically sweet in flavor and smell to attract organisms. When eaten, the seeds are not digested. Instead, the seeds pass through the organism’s digestive system, exiting in the form of excrement. Thus, seeds have the chance to grow in many new environments that the parent plant cannot reach. This increases the chance that at least some offspring will end up in a favorable environment and wil survive to pass on their genes to the next generation.
Finally, some angiosperms have fruits in the form of burrs that can cling to animal fur. Others have fruits that work like kites to be dispersed by the wind (or human kids playing with the "helicopters" :P). These also enable plants to disperse their seeds across a wide location in order to help ensure the survival of at least some offspring.
Interlude 1: Fun Facts about Plants! among other things, like mushrooms and cashews and stars and your brain and atoms and... a bunch of other stuff :)
Or the absence of an aquatic environment to support the plant body.
In terms of cells in general that are used for support, there are three main types: parenchyma, collenchyma, and sclerenchyma. Parenchyma cells have many functions. One such function can be to support the plant and allow it to grow taller. However, they are usually put to other purposes by the plant. Collenchyma cells are elongated thickened cells at the sub-epidermal level of the plants. Made up of cellulose and pectin, they can provide structural support the plant’s growing shoots and leaves. For example, celery is made up of collenchyma tissues. Some flowering plants (such as Oak trees) have woody tissues made of lignin that allow them to maintain rigidity and structure as they grow taller. These sclerenchyma cells support the plants because of their hard thick cell walls. Most die after each growing season, and new ones grow and develop each year.
Stems of plants are also made up of cells meant to help the plant grow taller. In particular, meristems are tissues in most plants that keep the plant growing because of their ability to divide rapidly. The apical meristems are completely undifferentiated tissues found at the buds and growing tips of roots in plants, meaning they still have no specific purpose for the plant. They begin growth of new cells.
Finally, true roots of plants allow for the base of the plants to remain stable as they grow taller and wider. They keep the plant steady either by growing shallow and widespread or growing deep into the ground. Even if the plants had a rigid stem, they would still fall over if they weren’t grounded properly by their roots. Thus, land plants had to develop these true roots.
Dirk Maassen - The Gate (Solo Piano Version) by Dirk Maassen
Interlude 2: How do plants grow?
Finally, flowering plants have to worry about dehydration.
A type of dermal tissue, called a cuticle, is the waxy covering on leaves and stems of plants that allows them to slow down the rate of water evaporation, thus helping them retain water. Land plants are at greater risk of drying out, so these cuticles help them survive on land by decreasing the need to absorb more water.
Because the cuticles work by preventing air from diffusing in and out of the plants, they also slow down cellular respiration and photosynthesis. Thus, land plants have small openings on the underside of their leaves. These openings, called stomata, open to let in oxygen and carbon dioxide, and they close to minimize water loss (by working to slow down evaporation like the cuticles). All land plants, not just flowering plants, have these adaptations.
On land, plants can only get water and minerals from the ground. They had to develop true roots to allow them to absorb water and minerals from the soil to survive. Made up of parenchyma cells, these roots also have root hairs that allow them to absorb even more water than usual. This works because with the increased surface area, plants can absorb more water faster. The parenchyma cells allow this because they have large central vacuoles, able to store much water (among other things). This water is soon transported by vascular tissues up the stem to the leaves of the plants.
Vascular tissues called the xylem carry water and nutrients up the stem to the leaves. It allows water to defy gravity and travel up the plant to replace water lost during transpiration and photosynthesis in the leaves. Because transpiration, evaporation, and photosynthesis take place at the leaves of the plant, there is lower pressure at the top of the plant and higher pressure near the roots. Water potential also plays a role in the way xylem works. Because pressures need to equalize, water is able to defy gravity and travel up the plant. This would reduce the pressure at the roots and increase pressure at the leaves.