XVII] POLLEN-TUBES AND STIGMA 169 



When the pollen-grain has reached the viscid surface of 

 the stigma it germinates there, and forms the pollen-tube. 

 The latter grows at the expense of the saccharine juices 

 excreted around it, and its apex at once bends down and 

 penetrates the stigma and style, the length of which it 

 traverses until it reaches the ovary ; here it passes along 

 the placenta and reaches the micropyle of an ovule, and 

 passes its contents over to the oosphere. 



The act of germination is exactly comparable to that 

 of any spore, and is started by the absorption of water 

 and food-materials, and oxygen, at a suitable temperature; 

 this act, and that of artificial germination in sugar- 

 solutions on glass slides, would alone suffice to indicate 

 that the pollen-grain is a true spore. 



The turning down and entrance of the tube into the 

 stigmatic tissues is facilitated by the as yet unexplained 

 attraction (chemotropism) exerted by food-materials on 

 the protoplasm, as Pfeffer and Miyoshi have shown for 

 fungus-hyphae and other cases ; and the further growth of 

 the tube in the tissues of the style takes place at the 

 expense of food-materials which it absorbs from the living 

 cells, just as the hypha of a parasitic fungus absorbs 

 them from the living cells of the leaf, stem, or other 

 organ in which it lives. 



In many cases (Crocus, Lily, &c.) the tissues of the 

 interior of the style are loose and open, affording a more 

 or less definite passage lined by thin-walled papillate 

 cells, and the tubes grow down in the slimy sugary 

 substances on their surfaces. Such papillose conducting 

 tissue is often continued down the walls of the ovary on 

 to the placentae, and in Orchids (e.g. Gymnadenia) it is 

 not difficult to trace the ends of the tubes to the three 

 parietal placentae: under a lens the tubes look like 

 unspun silk. 



