441 VEGETABLE PHYSIOLOGY 



normal ones. The most conspicuous instances of this are 

 afforded by several species of the genus Viola. In one of 

 these flowers the pollen grains often put out their pollen 

 tubes while they are in the sporangia, and the tubes grow 

 towards the stigma, penetrating it and reaching the ovules 

 as in the case of the normal flower, fertilisation resulting 

 in the same way. 



The process of pollination is followed in the ordinary 

 course of events by the germination of the microspore or 

 pollen grain. The facts that it grows upon the substratum 

 of the stigmatic surface and that the resulting gametophyte 

 or pollen tube is often of considerable length mark a great 

 difference between it and the gametophytes of the vascular 

 cryptogams. It becomes indeed a parasite feeding upon a 

 host plant during the greater part of its development. 



The course of events in the germination of the pollen 

 grain appears to be the following. At the outset it absorbs 

 water from the moist surface of the stigma and swells, its 

 protoplasm becoming generally more granular. It almost 

 simultaneously absorbs such food material as the surface 

 of the stigma can supply, usually some kind of sugar. 

 Most pollen grains contain a certain amount of reserve 

 food material, frequently starch or sugar, or both. The 

 process of absorption is followed by the secretion of 

 enzymes, which can act upon these reserve materials, the 

 most prominent of which are diastase and invertase. The 

 former seems to be the most widespread, but the latter is 

 far from uncommon. In some cases both enzymes are 

 developed. The outer coat of the grain then bursts, and 

 the inner one begins to protrude, probably in consequence 

 of the hydrostatic pressure set up by the water that has 

 been absorbed. Usually only one such tube protrudes, 

 though occasionally several are developed. Intra-cellular 

 digestion of the reserve materials follows, and the tube 

 grows at their expense. The increased nutrition is fol- 

 lowed by a further increase of the enzymes, which is 

 somstimes preceded by a temporary diminution. This, 



