THE PLANT: ITS STRUCTURE, LIFE-PRO CESSES AND ENVIRONMENT By Ir. J. V. Osterhout Plants resemble animals in their fundamental life-processes and in their essential requirements of food, air, water, warmth and light. But the green plants possess an important advantage over animals since they are able to manufacture food from air and soil-water. This process depends on the action of chlorophyll (leaf-green) in the sun light, by the absorption of which the necessary energy is supplied. Other differences between animals and plants, as that plants take up food in dissolved form only and have cellulose walls, are of minor importance.
The cell: protoplasm.
Plants are composed of cells of microscopic size, the outer walls of which are usually of cellulose (the substance of which paper is made). Figs. 23, 24 represent plant-cells. Within the non-living cell-wall is contained the living part, consisting of a transparent, jelly-like colloid substance called pro toplasm. Its principal constituents, besides water, which constitutes 80 to 90 per cent of the plant, are proteids (white of egg substance), fats and oils, sugars and various salts.
Protoplasm is able to build new living protoplasm from the lifeless materials at its disposal ; it can grow and reproduce ; it has the power of movement and of responding to stimuli. It conducts complex chemical processes (metabolism), by means of which the living substance is built up (constructive me tabolism) or torn down (destructive metabolism).
All the characters of the organism are an ex pression of the activity of its protoplasm. As long as certain and physical processes take place in tte protoplasm we say the organism is alive ; when these stop, we say that it dies. Such substances in the cell as enter into these processes we regard as living ; others, as starch grains, which take no part in them, we regard as dead. The latter may at any time enter into these processes, as when starch is converted into sugar, and so become part of the living substance. The transformation of lifeless into living substance, and vice versa, is constantly taking place. Protoplasm may be killed
in a variety of ways, as by electric shock, heat, light, mechanical injury or poisonous substances.
Within the protoplasm, or cytoplasm, of the cell is contained a body, usually spherical or ellipsoid in shape, called the nucleus. It contains a deeply staining substance called chromatin.
There is abundant evi dence that the heredi tary characters, those handed down from par ent to offspring, are somehow bound up in the chromatin, a n d that it is the union of chromatin from both parents in the act of fertilization which causes the offspring to partake of the char acters of both. It has been demonstrated that if the offspring receives protoplasm from both parents but chromatin from only one, it shows the characters of only that one.
The division of the cell is accompanied by a division of the nucleus, which may be either direct (amitosis) or indirect (mitosis). In direct division the nucleus constricts in the middle and the two halves simply pull apart. In indirect division the chromatin breaks up into a number of bodies (chromosomes), whose number is constant in each species. They arrange themselves on a spindle shaped body, known as the mitotic spindle, and each chromosome breaks into two, the halves going to opposite ends of the spindle and there forming daughter-nuclei. A cell-wall is formed midway between these, dividing the cell into two (Fig. 23).
Plant organs: structure and function.
The plant body is divided into root, stem and leaf. The structure of each of these organs is adapted to the work it performs. Structure and function will here be considered together.
The principal work of the root is to explore the soil for moisture. It is unerringly guided downward by gravity, which acts as a stimulus, causing the upper side of the root to grow faster than the lower side, hence forcing the tip downward, no matter how it be placed. Mois ture attracts the root very strongly ; roots have been found in cisterns as much as 200 to 300 feet from a tree.