THE PLANT: ITS STRUCTURE, LIFE - PROCESSES AND ENVIRONMENT 



11 



the place. Some societies seem to be largely accidental in population, however, and others seem to be 

 governed by definite laws or relationships. These laws of adaptation are very little understood. It is 

 now suspected that there may be positive physiological incompatibility between some kinds, and toler- . 

 ance, congeniality or even symbiotic relationships between others. Under some kinds of trees, for exam- 

 ple, certain kinds of herbaceous plants may thrive and others may perish, even when both are equally 

 exposed to sunlight : it is doubtful whether this difference is to be explained by competition for food or 

 moisture. We do not know why some weeds thrive in a corn-field and others do not. There may be 

 bacterial or other organic relations between some kinds. There may be root-excretions that are hurt- 

 ful to some plants and harmless or even useful to others. Perhaps the crop rotations that experience 

 has found to be useful are dependent in some measure on such vital relationships as these. 



THE PLANT: ITS STRUCTURE, LIFE-PRO- 

 CESSES AND ENVIRONMENT 



By W. 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 tal{e 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). 



Fig. 23. A plant cell. The figure shows the rotation of pro- 

 toplasm. (Blodea, or Anaoharis.) 



All the characters of the organism are an ex- 

 pression of the activity of its protoplasm. As long 

 as certain chemical and physical processes take 

 place in tj.e protoplasm we say the organism is 

 alive ; when these stop, we say that it dies. Such 

 substances in the cell as enter into theseprocesses 

 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 



Fig. 24. Common ceU forms. The 

 walls are thickened at the 

 angles, forming strengthen- 

 ing tissue. This kind of tis- 

 sue is known as eollenchyma 

 (page 8). 



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-^s^ 

 the chromatin, and"\i 

 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. 25). 



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 root. — 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 



