EEPEODUCTION 455 



nutrition is followed by a further increase of the enzymes, 

 which is sometimes preceded by a temporary diminution. 

 This, however, does not last long, and soon a considerable 

 increase can be observed. In some pollen-tubes such as those 

 of the Lily, in whose pollen starch granules are abundant, 

 the process of the digestion of the starch can be observed 

 taking place as the granules move along the tube during 

 its elongation. Soon an excretion of the enzyme into the 

 tissues of the style takes place, and the reserve materials 

 which are stored in the style are gradually digested as the 

 tube advances, thus ministering to its nutrition, absorption 

 of the products of the digestion being effected by the tube. 

 The latter in most cases makes its way to the micropyle of 

 the ovule, and by this channel reaches the embryo-sac or 

 megaspore. At this period the latter contains its gameto- 

 phyte, or prothallium, at the apex of which the oosphere 

 or female gamete occurs. The tip of the pollen-tube comes 

 in contact with the wall of the embryo-sac close to the 

 oosphere. It then contains two gametes, which are un- 

 differentiated masses of protoplasm, each with a vry large 

 nucleus. The separating walls become deliquescent and 

 are absorbed, and one of the male gametes fuses with the 

 oosphere, forming as before a zygote, while the other often 

 fuses with the definitive nucleus, as has already been 

 described. 



In a few cases the pollen-tube makes its way to the base 

 of the embryo-sac and burrows through its contents, reach- 

 ing the oosphere from below. This has been observed 

 particularly in Casuarina and in certain of the forest trees. 



A few variations of this process have been observed 

 among the Gymnosperms. As the plants of this group are 

 all diclinous, self-pollination is of course impossible. The 

 agent of pollination is usually the wind, and the pollen- 

 grain in these plants falls upon the micropyle of the ovule, 

 there being no ovary and consequently no stigma. The 

 growth of the tube is slow, sometimes extending over 

 several months. Indeed. in some cases the sporangium is 



