VIII.] THE BEAN-PLANT. 69 



nucleus. The nucleus contains a sac, the embryo sac, in 

 which certain cells, one of which is the embryo cell, and 

 the rest endosperm cells, are developed. A pollen grain 

 deposited on the stigma, sends out a hypha-like prolongation, 

 the pollen tube, which elongates, passes down the style, and 

 eventually reaches the micropyle of an ovule. Traversing 

 the micropyle, the end of the pollen tube penetrates the 

 nucleus, and comes into close contact with the embryo sac. 

 This is the process of impregnation, and the result of it 

 is that the embryo cell divides and gives rise to a cellular 

 embryo. This becomes a minute Bean-plant, consisting of a 

 radicle or primary root ; of two, relatively large, primary 

 leaves, the cotyledons; and of a short stem, the plumule, on 

 which rudimentary leaves soon appear. The cotyledons now 

 increase in size, out of all proportion to the rest of the em- 

 bryonic plant ; and the cells of which they are composed be- 

 come filled with starch and other nutritious matter. The 

 nucleus and coats of the ovule grow to accommodate the en- 

 larging embryo, but, at the same time, become merged into an 

 envelope which constitutes the coat of the seed. The pistil 

 enlarges and becomes the pod ; this, when it has attained its 

 full size, dries and readily bursts along its edges, or decays, 

 setting the seeds free. Each seed, when placed in proper .con- 

 ditions of warmth and moisture, then germinates. The cotyle- 

 dons of the contained embryo swell, burst the seed coat, and, 

 becoming green, emerge as the fleshy seed leaves. The nutri- 

 tious matters which they contain are absorbed by the plumule 

 and radicle, the latter of which descends into the earth and 

 becomes the root, while the former ascends and becomes the 

 stem of the young bean-plant. The apex of the stem retains, 

 throughout life, the simply cellular structure which is, at 

 first, characteristic of the whole embryo; and the growth 

 in length of the stem, so far as it depends on the addition of 

 new cells, takes place chiefly, if not exclusively, in this part. 



