THE SPERMAPHYTES. 



219 



FIG. 137. a, a pollen grain of the Angiosperm, Allitnn Jistulosum ; b and c 

 show two stages of the development of the tube ; d and e show two stages 

 in the development of the pollen grain of Monotropa hypopitys ; f, a num- 

 ber of pollen grains of Platanthera bifolia in process of division ; g, pollen 

 grain of Orchis mascula forming its tube. (After Strasburger.) 



ments worked by nature, are carried by 

 or in some similar manner are brought 

 of the same or of different flowers in 

 contact with the viscid fluid secreted by 

 the stigma, which forms the tip of the 

 pistil. Excited into activity by this 

 fluid, .the pollen grain bursts its outer 

 coat and expands its inner coat into a 

 tube which grows down through the 

 stigma and style until it comes in con- 

 tact with the ovule, enters the micro- 

 pyle of the ovule, and finds the embryo 

 sac. The contents of the pollen grain 

 now mingle with those of the embryo 

 sac, and the life of a seed commences. 

 (See Figs. 137 and 138.) 



The matured seed incloses an embryo 

 and more or less stored-up material, the 

 endosperm, to nourish the embryo when 

 it germinates. The relative proportions 

 of embryo and endosperm vary greatly 



insects or birds, 

 from the anthers 



FIG. 138. Longitudi- 

 nal section of an 

 ovary of an An- 

 giosperm. , the 

 stigma ; b, pollen 

 grains ; c, a pollen 

 tube passing down 

 to the egg cell, se. 

 (After Thome and 

 Bennett.) 



