Page:The American Cyclopædia (1879) Volume XIII.djvu/590

 574 PLANT FIG. 2. Plant of Elon- formed, and finally the segments separate, each portion growing to the full size and repeating the operation of subdivi- sion. Thus in a simple microscopic cell there is nutrition, growth, and re- production. A step high- er in organization are found plants of numerous simple cells placed end to end, as seen in some mi- nute alg89 and fungi, in which a similar subdivi- sion of the cells takes place, accompanied by a growth of the parts pro- duced by division and elongating the chain of cells. In some of these microscopic algaB the co- operation of two distinct cells is necessary to repro- duction. A phenomenon called conjugation may be observed in some, such as zygnema, a very slender silk-like alga which forms a green scum of fine threads on pools in spring ; a microscope of moderate power will show these P ] f * * " nsist f <*- gated transparent cells placed end to end, wi'thin which the protoplasm is distinctly visible. The cells of two adjacent threads may be seen to bulge out toward one another ; at length the projecting por- tions of the cells meet and finally touch, the cell wall between them is broken, and the contents of one cell pass over into the other, and mingling with the protoplasm of that cell form a spore, or reproductive body analogous to a seed, which being liberated starts life anew. From this sim- ple union of two dif- ferent cells to form a reproductive body, it is but a step for the plant to have special cells devoted to re- production, while the other cells of the plant are engaged in pro- moting g i g ts growth. The largest plant is made up of minute cells, each of which at some stage of its existence has passed through a life similar to that described for the pro- tococcus ; it has been formed by the division of other cells, and has grown to its full size. Germination and Growth from the Seed. A general view of plant life, as manifested in the higher forms of vegetation, may be best given by following a plant from the begin- ning. The first rudiment of the plant is to be found in the ovule or the forming seed. The ovule (the nature and structure of which will be explained further on) is the small pulpy body within the ovary which is to become the seed vessel, and which is itself to become the seed ; a cell within the ovule is the beginning of the plant; this elongates by subdivision, and also increases in width until, taking the common bean as an example, two thick lobes are formed, connected by a short stem ; when these parts are perfectly formed all growth ceases, and the seed is ripe. If we soak a ripe bean in warm water until the skin can be re- moved, the parts may be seen, two lobes and a small stem uniting them, which in the bean is bent ; this part of the seed (and in the bean it is the whole of it besides the seed coats) is the embryo. This is formed in every seed, though often much less distinctly than in the bean, and sometimes with only one lobe in- stead of two. The seed is to all appearance perfectly lifeless, dry, often very hard, and even bony, and it may remain dormant for years; some have been known to germinate after being kept in the ordinary way for half a century, and even after several hundred years when deeply buried in the soil ; but the sto- ries told of seeds taken from mummies sev- eral thousand years old and germinating have no foundation in fact. The essential condi- tions to germination are moisture, air, and heat ; these must be present in proper propor- tions, and are best applied through the me- dium of the soil, though germination will take place when these conditions are supplied in other ways. The first step is the absorption of moisture, which must be present in suffi- cient quantity, but for land plants there must not be an excess or the seed may rot ; besides this, too much water excludes the air, without the oxygen of which germination is not possi- ble. It was formerly supposed that darkness was essential, but experiments have shown that germination is not influenced by the presence or absence of light. The temperature required varies with different seeds ; while some will germinate at a few degrees above the melting point of ice, others, seeds of tropical plants, require 100 F. or more for their most suc- cessful germination ; there is a degree of heat most favorable for each kind of seeds, though they will start at a much lower and will en- dure a much higher temperature ; thus, while Indian corn will germinate at 48 and tolerate a heat of 115, it is found that the proc goes on most rapidly at the temperature of 93. The time required for germination, evei when the conditions are most favorable, varu