CHAP. XVI.] THE BEAN PLANT. 461 



invested by two coats, an outer and an inner. Opposite 

 the surhmit of the nucellus, these coats are perforated by 

 a canal, the micropyle, which leads down to the nucellus. 

 One of the cells of the nucellus is very much larger than 

 the rest ; this is called the embryo-sac. In its interior several 

 smaller cells are developed, the most important of which 

 is the ovum or oosphere, which lies at the end of the embryo- 

 sac towards the micropyle. When the pollen is deposited 

 on the stigma the grains germinate. Each sends out a long 

 filament, 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 pene- 

 trates the nucellus, and comes into close contact with the 

 embryo sac. The original nucleus of the pollen-grain has 

 in the meantime divided into two, and one of the daughter 

 nuclei divides again. All three pass down the tube, one 

 nucleus reaches the embryo-sac, and then fuses with the 

 nucleus of the ovum. This is the process of impregnation, 

 and the result of it is that the ovum 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 embryonic plant ; and the cells of which 

 they are composed become filled with starch and other 

 nutritious matter. The nucellus 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 



