CHAP. XVI.] THE BEAN PLANT. 461 



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

 the summit 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 ertibryo-sac. In its interior several 

 smaller cells are developed, the most important of Avhich 

 is the oviwi or oosp/iere, 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 mean time divided into two, and one of the daughter 

 nuclei passes down the tube, 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 in- 

 crease 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 

 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 warmth and moisture, then germinates. 



