444 VEGETABLE PHYSIOLOGY 



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 

 undifferentiated masses of protoplasm, each with a very 

 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. The fate of the 

 other male gamete is uncertain ; it has generally been 

 held to disintegrate and disappear, but in Lilium Martagon 

 it has recently been seen to fuse with one or both of the 

 polar nuclei which go to make up the definitive nucleus of 

 the embryo-sac. How far this phenomenon is of general 

 occurrence is not known. 



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



. base of the embryo-sac and burrows through its contents, 



reaching 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. 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 detached from the parent 

 plant before it has reached the embryo-sac, from which 

 it is separated by a bulky portion of the nucellus or body of 



