282 MORPHOLOGY OF GYMNOSPERMS 



(86, 132, 157), that is, the uninucleate spore is shed; but in both 

 these genera the division to form generative and tube nuclei occurs 

 on the nucellus, before tube-formation. Pollination at a later stage 

 has been reported for Picea excelsa (79), in which the next division 

 has occurred, the generative cell having produced the stalk and body 

 cells. In Widdringtonia (159, 176) the divisions are remarkably 

 delayed. The spore is shed in the uninucleate condition, and since 

 there are no prothallial cells there has been no division. Unlike 

 Cupressus and Juniperus, however, whose pollen grains are shed in the 

 same condition, the nucleus does not divide until after tube-formation, 

 the generative and tube nuclei being observed only in the tubes. The 

 single pollen tube penetrates the megaspore membrane and passes 

 down inside of it for one-third to one-half the length of the gameto- 

 phyte, and only after entering within the membrane does the genera- 

 tive cell divide. When the tube has reached the limit of its growth, 

 its tip may contain only the large body cell, the stalk and tube nuclei 

 having a tendency to disappear. Two of the numerous archegonia 

 are fertilized by the two male cells. 



The period that elapses between pollination and fertilization, which 

 covers the activity of the pollen tube and the life of its contents, falls 

 into two categories. One of them includes cases in which pollination 

 occurs one season and fertilization the next. As has been mentioned 

 (p. 275), this is true of Pinus Laricio, in which pollination occurs in 

 June and fertilization about July first of the following year. It is 

 also true of Juniperus communis (132, 157, 173), which has about 

 the same period; and in both cases the generative cell does not divide 

 until the second season. In Sciadopitys, however, Lawson (175) 

 reports that the generative cell divides during the first season into 

 the body cell and a free stalk nucleus, and that the division of the 

 body cell and fertihzation do not occur until the second season. In 

 Widdringtonia, also, Saxton (176) reports that fourteen or fifteen 

 months elapse between pollination and fertilization. In many other 

 forms approximately the same periods have been observed. These 

 long periods may escape recognition unless there is familiarity with 

 the condition of adjacent structures and especially of the female 

 gametophyte, for two similar pollen tubes may belong to two different 

 seasons. 



