54 THE GRASSES 



perforated by a canal, the micropyle, which leads down to 

 the nucleus. The nucleus contains a sac — the embryo sac— 

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

 the rest the endosperm cells, are developed. A pollen grain 

 deposited on the stigma sends out a thread-like prolonga- 

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

 and eventually reaches the micropyle of an ovule. Tra- 

 versing 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 cellu- 

 lar embryo. This becomes a minute bean plant, consisting 

 of a radicle or primary root; of two relatively large pri- 

 mary leaves, the cotyledons ; and 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 

 embryonic plant ; and the cells of which th&y are composed 

 become filled with starch and other nutritious matter, 

 legumin). 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 tree. 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 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 tissues which compose the body of the bean plant are 

 bounded at the surface by a layer of epidermic cells, within 

 which, rounded or polygonal cells make up the ground 

 substance or parenchyma of the plant, extending to its very 



