STRUCTURAL BOTANY. 63 



branous sac, or cell, filled with a fluid. Its membrane, or 

 coat, is double, the outer layer thick, but weak, the inner 

 one very thin, but distensible, the former usually being 

 striped or banded. The thickish fluid within contains 

 minute molecular granules. When pollen-grains are ex- 

 posed to moisture, they swell to such a degree that many 

 kinds burst and discharge their contents. In the As- 

 clepiads and Orchids the pollen-grains cohere in masses, 

 (i^WQ^jpoIlinia. (PL XIII., 12 J.) 



115. Of (be Eepkoductiye Organs, the pistils and the 

 stamens, the latter are adapted to fertilize, the former to be 

 fertilized. The pollen-grains, after being brought in con- 

 tact with the stigma, are retained there by its loose papillge 

 or projecting hairs. Having absorbed moisture from the 

 stigma, they soon begin to grow or germinate. The inner 

 coat of the grain expands and breaks through the outer 

 one, which is weak and brittle, in the form of a tube, filled 

 with the liquid and molecules, contained in the cavity of 

 the grain. This tube penetrates like a radicle the soft 

 tissue of the stigma, and, growing downward into the 

 style, reaches the placenta, where, after entering the mi- 

 cropyle of an ovule (whether solitary, or one of many), it 

 makes its way to the surface of the embryo-sac^ but prob- 

 ably not farther (see figures of Cut XII. : Fig. 1, ver- 

 tical section of the pistil of Buckwheat, enlarged ; 6>, the 

 ovule with its two coats ; ^, the embryo-sac ; i;, the em- 

 bryonic vesicle ; ^, a pollen-tube, having entered the mi- 

 cropyle ; f\ a pollen grain with its tube, separate). 



It must be remembered that the embryo-sac at this 

 stage as yet contains no embryo (§ 107). 



The pollen-tube, having reached the surface of the 

 embryo-sac, in some unexplained manner, causes the for- 

 mation w^ithin it ol the emhryonic vesicle^ which is attached 

 to its wall next the micropyle (Fig. 2, the embiyo-sac with 



