As with most things, there were advantages and disadvantages to living on land.
One advantage is that light is more abundant. Another is that there is continous movement of Carbon Dioxide and Oxygen used in photosynthesis and respiration.
The disadvantages aree that plants can not simply release reproductive cells into the water, there wasn't always a supply of water, water will no longer flow freely through them, they don't have anything to support them, and nutrients will not simply flow into them.
To get water, and to anchor plants into the ground, they developed roots of one of two types. Water is taken into the roots through osmosis. To get water up small plants (herbaceous), capillary action is sufficient. but in larger plants (woody), it was not enough to get water up against the pull of gravity. These plants developed xylem and phloem, which transport water and nutrients respectively, throughout the plant.
Xylem's main task is carrying water and dissolved nutrients from roots to stems and leaves. They also provide strength to the woody parts of large plants because of their thick cell wall. Strangely, most xylem cells grow to maturity and die before they function as water carriers.
Phloem's task is to carry the products of photosynthesis and certain other substances throughout the plant. While xylem carries things upward only, phloem cells can carry things up and down the plant. Phloem cells are alive and filled with cytoplasm when they are functioning, unlike xylem cells.
There are two types of seed plants: angiosperms, and gymnosperms. Gymnosperms evolved for wet climates, and angiosperms evolved for dry climates.
Gymnosperms have leaves which have evolved into specialized reproductive structures called scales, which are grouped into cones. Male cones produce pollen, and female cones produce eggs. The developing seeds are held on the scales of female cones. The seeds are protected by seed coats, but not by the cone. For this reason, they are called naked seed plants. (gymno- means naked, and -sperm means seed.)
The most basic difference between moncots and dicots is that monocots have one seed leaf, and dicots have two. This is easy to remember, because mono means "one", di means "two", and cot is short for cotyledon, which is a seed leaf. Cotyledons are filled with food for the embryo in some species, and are the first to perform photosythesis in other species.
|Flower||flower parts in multiples of 3||flower parts in multiples of 4 or 5|
|Leaves||veins parallel||veins branch out|
|Stem||don't grow thicker||can grow thicker|
|Vascular bundles in roots||xylem and phloem alternate in circle||xylem forms "X", phloem is between arms of "X"|
|Vascular bundles in stem||scattered||arranged in a ring|
The male gametophyte of seed plants is contained in a pollen grain. Instead of the sperm swimming through water, the entire pollen grain is taken to the female gametophyte by wind, insects, birds, small animals, or bats. Once the male gametophyte reaches the female gametophyte, the seed is fertilized. This process is known as pollination.
After fertilization, the zygote protected by the seed grows into a tiny plant called an embryo. The embryo stops growing while it is still quite small. Later when the embryo begins to grow again, is will use a supply of stored food inside the seed. A seed coat surrounds the embryo and protects it and the food suply from drying out. In the seed coat, the embryo can remain dormant for many years. Seeds can survive long periods of cold, heat, or drought, and then grow only when conditions are right. Thus, seeds give these plants a major advantage over other plants.
For flowering plants, pollination is essential. Without it, they would not be able to reproduce. Some plants are pollinated by the wind, although this wastes most of the pollen. These plants usually have small, plain flowers with little or no fragrance.
Most angiosperms, though, are pollinated by insects, birds, or mammals that carry pollen from one flower to another, and are "paid" with a little food. The food may be pollen or nectar, which may be up to 25% glucose.
This interaction between plants and animals benifits both. Animals get food, and plants benefit because vector pollination is far more efficient that wind pollination. As a result, coevolution of plants and animals began accidentally.
Flowers pollinated by bees have markings that are only visible in ultraviolet light, which bees can see. They have also developed some sort of landing platform, because bees only gather nectar when they are standing.
Flowers pollinated by moths are often plain and white, but extremely fragrant, because moths have poor vision, but an excellent sense of smell. These flowers usually don't have landing platforms, because moths feed while hovering in midair, and use their long tongues to get the nectar which is usually contained deep within these flowers.
Some flowers produce a smell of rotting meat, to attract flies which lay their eggs in dead animals. They go so far as to heat up when they are ready to be pollinated, intensifying the smell, and increasing their chances of vector pollination.
Just as flowers have adaptations to ensure pollination, fruits have adaptations to scatter the seeds away from the parent plant. This seed dispersal is important because if the seed fall to the ground next to the parent plant, they will be competing for food, water, and light, which will reduce the chances of survival for the growing seeds. This also allows plants to colonize new environments.
Some seeds are carried by the wind, such as cottonwood or dandelions. Some abgiosperms build pressure in the fruit until the seed is fired like a bullet. Some plants have fruits that spring open when they are touched, scattering the seeds over signifigant distances.
Meristematic tissue is the only growth tissue in plants. It divides rapidly, through mitosis, and has thin cell walls. The new cells look alike at first, but differentiate into tissues of various kinds as they mature.
There are several places meristematic tissue is found. The apical meristem at the end of each growing stem and root, which enables the stems and roots to grow in length. They continue to grow out from their tips until they die. This is known as an open type of growth, and plants are the only things that have this type of growth. In comparison, all animals stop growing when they reach a certain size.
Cork cambium and vascular cambium, two other kinds of meristematic tissue, allow branching out of roots and stems, and well as allowing them to grow thicker. Vascular cambium produces vascular tissues and increases the stem thickness over time. Cork cambium produces the stem's outer covering. Pericycle, another kind of cambium, is found in roots. It enables roots to branch and grow thicker.
Water is absorbed by root through osmosis. The root hairs increase the surface area of the roots, allowing them to absorb more water. Xylem carrys the water up the stem to the leaves, where it is used for photosynthesis. Because the plant is constantly using water during daylight, it can absorb quite a bit of water. The roots also bring in nutrients through active transport.
Taproots are large main roots with secondary roots coming off of it. The main root grows longer and thicker, but the secondary roots remain small.
Fiberous roots are many, but all are the same size. ("All roots are created equal.") Secondary roots grow and branch, but get no larger than the other roots. These root systems can have as many as 14 million secondary roots. These extensive roots systems are key in holding topsoil so it doesn't get washed away by rain.
Roots serve three important purposes: "grocery shopping", "warehouse", and "anchor".
The grocery shopping is collecting water and nutrients from the soil around them. Water is taken in through osmosis, while nutrients are brought in through active transport. Because water is constantly (during daylight) being carried up the stem by the xylem, to the leaves which use it for photosynthesis, it can be absorbed in large quantities.
Some roots are able to store water or food. This is the "warehouse" function.
The roots also hold, or anchor, the plant to the ground. The stem would not be able to stand upright if it was not anchored by the roots.
Some roots are also able to regenerate the plant if the stem is broken off. This is why many weeds are difficult to get rid of.
Stems can vary greatly in size and shape, but all stems serve tow important purposes: They hold the leaves up to the sun, and carry materials between the roots and the leaves.
There are four main tissue types in stems: parenchyma tissue (pith), vascular tissue (xylem and phloem), cambium tissue (vascular cambium and cork cambium), and cork tissue (outer bark). Woody plants also have a core of dead xylem cells which support the tree.
Some stems are modified for storage of water or food. These would be rhizomes, tubers (such as potatoes), bulbs (such as onions and some flowers), and corms. All these stems are doing is swelling full of starch or water.