130 



VEGETABLE LIFE AND WORK. [SECTION 16. 



forjn a compound mass, eacli 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. While this growth was going on, each grain of the 

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

 parent, of something different from protoplasm, — something which hardly 

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

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

 433 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- 

 |-4t| 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, wliich is common 

 in early spring, requires only moderate magnifying power 

 to bring to view what is shown in Pig. 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. O 

 A transient inspection shows, what prolonged 

 watching would confirm, that each sphere di- ^ O 

 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- @0 

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

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

 protoplasm within; through it the latter imbibes sur- Q^ ^Q 

 rounding moisture, which becomes a watery sap, occupy- 

 ing vacuities in the protoplasmic mass which enlarge or 

 lun together as the periphery increases and distends. 

 When full grown the protoplasm may become a mer« lining 

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

 the waU-Kning by delicate threads of the same substance. So, when full 

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



437 



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

 embryo ; a single mass of protoplasm (Fig. 433) dividing into two, three, and then 

 into more incipient cells, which by continued multiplication build up an embryo. 



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

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



