130 



VEGETABLE LIFE AND WORK. [SECTION 16. 



433 



form a compound mass, each grain or globule of which goes on to double 

 itself as it grows ; and the definite shaping of this still increasing mass 

 builds up the embryo into its form. 



399. Cell-walls. \Vhile this growth was going on, each grain of the 

 forming structure formed and clothed itself with a coat, thin and trans- 

 parent, of something di lie rent from protoplasm, something which hardly 

 0and only transiently, if at all, partakes of the life and action. 

 The protoplasm forms the living organism; the coat is a kind 

 of protective covering or shell. The protoplasm, like the 

 flesh of animals which it gives rise to, is composed of four 

 chemical elements: Carbon, Hydrogen, Oxygen, and Nitro- 

 gen. The coating is of the nature of wood (is, indeed, that 

 which makes wood), and has only the three elements, Car- 

 bon, Hydrogen, and Oxygen, in its composition. 



400. Although the forming structure of an embryo in 

 the fertilized ovule is very minute and difficult to see, there 

 are many simple plants of lowest grade, abounding in pools 

 of water, which more readily show the earlier stages or sim- 

 plest states of plant -growth. One of these, which is common 

 in early spring, requires only moderate magnifying power 

 to bring to view what is shown in Fig. 437. In a slimy 

 mass which holds all loosely together, little spheres of green 

 vegetable matter are seen, assembled in fours, 

 and these fours themselves in clusters of fours. 

 A transient inspection shows, what prolonged 

 watching would confirm, that each sphere di- 

 vides first in one plane, then in the other, to 

 make four, soon acquiring the size of the original, and so on, 

 producing successive groups of fours. These pellets each 

 form on their surface a transparent wall, like that just des- 

 cribed. The delicate wall is for some time capable of expan- 

 sive growth, but is from the first much firmer than the 

 protoplasm within; through it the latter imbibes sur- 

 rounding moisture, which becomes a watery sap, occupy- 

 ing vacuities in the protoplasmic mass which enlarge or 

 run together as the periphery increases and distends. 

 When full grown the protoplasm may become a mere lining 

 to the wall, or some of it central, as a nucleus, this usually connected with 

 the wall-lining by delicate threads of the same substance. So, when full 

 grown, the wall witli its lining a vesicle, containing liquid or some 



Fio. 433-436. Figures to illustrate the earlier stages in the formation of an 

 eml.ryo; a single mass of pmtoiilasm (Fig. 433) dividing into two, three, and then 

 into more incipient cells, which l>y continued multiplication build up an embryo. 



Fie. 437. Magnified view of some of a simple fresh water Alga, the Tetraspora 

 lubrica, each .sphere of which may answer to an individual plant 



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