VIIL] . THE BEAN-PLANT. 71 



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 prolonga- 

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

 and eventually reaches the micropyle of an ovule. Travers- 

 ing 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 give 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 

 become filled with starch and other nutritious matter. The 

 nucleus and coats of the ovule grow to accommodate the 

 enlarging 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 conditions of warmth and moisture, then germinates. 

 The cotyledons of the contained embryo swell, burst the 

 seed coat, and, becoming green, emerge as the fleshy seed 

 leaves. The nutritious matters which they contain are ab- 

 sorbed by the plumule and radicle, the latter of which de- 

 scends 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 



