Whereas most organisms are dependent on other organisms for energy, plants can capture energy directly from sunlight and convert it into a usable form through the process called photosynthesis.
Photosynthesis is the route by which virtually all energy enters life on Earth as we known it. Basically, photosynthesis is the conversion of water and carbon dioxide into glucose, water and oxygen. Water is also, like oxygen, a by product of photosynthesis which is then released into the atmosphere. Glucose is used as the main source of carbon from which plant use to make other molecules they and ultimately we need. For this reason, plants are referred to as autotrophs, i.e. the producers in a habitat opposed to us and all other beings known as heterotrophs, the consumers.
To that end, plants developed special features and strategies which allow them to obtain water and the mineral nutrients that they are not capable of synthesizing on their own. Plants have thus to evolve and adapt to very strange environments and climates, many of which quite adverse to plant life.
Basically, the plant body is quite similar in structure in all species, from the gigantic Mountain Ash to the minuscule cushion plants of Tasmania, even if at first they seem quite dissimilar. Some structures are absent in some species as result of adaptations during their evolution: e.g. some aquatic plants lost their roots as the minerals that they need are dissolved in the sea water they live in; some parasitic plants have no leaves as they get all their nutrients from their hosts and therefore no longer need leaves to photosynthesize.
But, back to the normal picture, we can say that a typical plant consists of one or several stems protruding from the soil where a root is anchored. Typically, the stem is further divided into branches from which many leaves and, at special times, flowers pop up. As many of us know, photosynthesis occurs in the leaves in which special openings called stoma (pl stomata) control the exchange of carbon dioxide, water and oxygen with the atmosphere. Water together with minerals dissolved in it is absorbed by the root tips through special cells. Linking the two there the stem or stems, or trunks in the case of trees, that supports the whole plant body above ground as serves also as a reserve organ for future times.
Usually, the plant dimensions bellow ground are as immense as those above it. Roots can “travel” indeed great distances to obtain water. In order to live best with what they get, plants developed special features or strategies to adapt to changing climates and conditions.
For example, to cope with water stress, mainly prolonged drought, some plants reduced greatly the size and number of their leaves. The extreme, of this can be seen in many cacti in which the leaves have turn into spiny structures used for defense purposes mostly.
In cacti, photosynthesis is thus made in the stem which also serves as a water storage organ. This why cacti have big, green, and round stems. In addition, to be most efficient on photosynthesizing cacti, as some other plant species from arid climates, developed as specialized chemical processes, along with cellular modifications, that make to conduct photosynthesis at night and not during the day. This way, they can save water by keeping their stomata closed during the day when humidity is lower. On the other extreme, we have mangroves where plants are faced with prolonged flooding, i.e. to much water. What to do then?
Well, like our cells, plant cells and specially the cells from plant roots need oxygen. As the concentration of oxygen in the water is much smaller than what exists in the atmosphere, plants in mangroves developed structures in their roots called pneumatophores specialized in capturing oxygen from the atmosphere. Also, salt concentration in mangroves is higher than what most plants face and certainly higher than their needs. To cope with these many plants developed special glands that excrete salt while others developed high salt tolerance that allowed them to compete and survive best in such conditions.
These are just extreme examples of myriads of adaptations that plants developed in order to conquer and survive on land.