140 



HOW PLANTS GROW. 



[LESSON 22. 



385. The pollen (297) which falls upon the stigma grows there 

 in a peculiar way : its delicate inner coat extends into a tube (the 

 pollen-tube), which sinks into the loose tissue of the stigma and 

 the interior of the style, something as the root of a seedling 

 sinks into the loose soil, reaches the cavity of "the ovary, and at 

 length penetrates the orifice of an ovule. The point of the pollen- 

 tube reaches the surface of the embryo-sac, and in 

 some unexplained way causes a particle of soft pulpy 

 or mucilaginous matter (Fig. 328) to form a mem- 

 branous coat and to expand into a vesicle, which is 

 the germ of the embryo. 



386. This vesicle (shown detached and more mag- 

 nified in Fig. 329) is a specimen of what botanists call 

 a Cell. Its wall of very delicate membrane encloses a 

 mucilaginous liquid, in which there are often some 

 minute grains, and commonly a larger soft mass 

 (called its nucleus). 



387. Growth takes place by this vesicle or cell, 

 after enlarging to a certain size, dividing by the for- 

 mation of a cross partition into two such cells, co- 

 hering together (Fig. 330) ; one of these into two 

 more (Fig. 331); and these repeating the process 

 by partitions formed in both directions (Fig. 332); 

 forming a cluster or mass of cells, essentially like the 



first, and all proceeding from it. After increasing in number for 

 some time in this way, 333 

 and by a continuation of 

 the same process, the em- 

 bryo begins to shape it- 

 self; the upper end forms 

 the radicle or root-end, 

 while the other end shows a notch between two lobes (Fig. 333), 

 these lobes become the cotyledons or seed-leaves, and the embryo 

 as it exists in the seed is at length completed (Fig. 336) 



FIG. 329. Vesicle or first cell of the embryo, with a portion of the summit of the embryo- 

 sac, detached. 330. Same, more advanced, divided into two cells. 331. Same, a little far. 

 tlier advanced, consisting of three cells. 332. Same, still more advanced, consisting of a 

 little mass of young cells. 



FIG. 333. Forming embryo of Buckwheat, moderately magnified, showing a nick at th 

 end where the cotyledons are to be. 334. Same, more advanced in growth. 335. Same, 

 still farther advanced. 336. The completed embryo, displayed and straightened out; the 

 same as shown in a section when folded together in Fig. 336. 



