12 ELEMENTS OF STRUCTURAL BOTANY. 



furnished with two coats, the inner one extremely thin, 

 and the outer one much thicker by comparison. The 

 interior of the pollen-grain is filled with liquid matter. 

 When a pollen-grain falls upon the moist stigma it 

 begins tn (/roiv in a curious manner. (Fig. 17). The 

 inner coat pushes its way through the outer one, 

 at some weak point in the latter, thus forming the 

 beginning of a slender tube. This slowly pene- 

 trates the stigma, and then extends itself down- 

 Fig. i7. wards through the style, until it comes to the 

 cavity of the ovary. The liquid contents of the pollen- 

 grain are carried down through this tuloe, which remains 

 closed at its lower end, and the body of the grain on the 

 stigma withers away. 



The ovary contains an ovule, which is attached by 



one end to the wall of the ovary. The ovule consists of 



a kernel, called the nucleus, which is usually surrounded 



by two coats, through both of which there is a 



minute opening to the nucleu's. This opening 



is called the mirropylc, and is always to be 



Fig 18. found at that end of the ovule which is not 



attached to the ovary. (Fig. 18, m.) 



About the time the anthers discharge their pollen, a 

 little cavity, called the enihr^/o-s>ic, appears inside the 

 nucleus, near the micropyle. The pollen-tube, with its 

 liquid contents, enters the ovary, passes through the 

 micropyle, penetrates the nucleus, and attaches itself to 

 the outer surface of the embryo-sac. Presently the 

 tube becomes empty, and then withers away, and, in 

 the meanwhile, a minute body, which in time developes 

 into the embryo, makes its appearance in the embryo- 

 sac, and from that time the ovule may properly be 

 called a seed. 



