CONIFERALES 313 



first prothallial cell. Sometimes the interval between the formation 

 of the second prothallial cell and the generative cell is long enough to 

 have the second prothallial cell slightly overgrown by the intine. 

 Where the generative cell divides before pollination, as in Abies, 

 the body cell becomes very large. Pollen grains, in all the conifers, 

 have a rich supply of starch. 



The body cell does not divide until the pollen tube grows down 

 into the nucellus. Its division, in nearly all cases, gives rise to only 

 two gametes. The male gamete is a short-lived cell. Consequently 

 one expects to find fertilization soon after this division. 



Just at the time of pollination, in nearly all cases, a pollination 

 drop appears on the tip of the ovule. It is colorless, and looks like a 

 small drop of glycerine. The pollen, falling on this drop, is drawn 

 down to the nucellus, where the growth of the pollen tube begins. 

 In the Araucariaceae the pollen lodges on the ovuliferous scale, or on 

 the ligule, or in the axil of the scale. In Larix and Pseudotsuga the 

 pollen does not reach the tip of the nucellus, but finds a lateral posi- 

 tion. There is a somewhat similar situation in Arthrotaxis and Se- 

 quoia. In such cases, the pollen tube grows somewhat laterally 

 through the nucellus to reach the egg; while in most cases, as in 

 Pinus, the growth is straight down, from the tip of the nucellus to 

 the egg. 



Dates of pollination and fertilization. — It is interesting to note the 

 time at which various stages in the life-history may be found. While 

 the time at which a certain stage, like pollination, takes place, may 

 vary with latitude and other factors, the intervals between stages 

 do not vary so much. Consequently, if the date of a certain stage is 

 known, the dates for other stages can be predicted approximately. 



In general, the interval between pollination and fertilization in 

 gymnosperms is longer than in angiosperms. Ephedra, with an in- 

 terval of 10 hours, being a notable exception. On the other hand, 

 there are a few long intervals in angiosperms, the longest being in the 

 oaks, where the interval is more than a year. Other members of the 

 Fagaceae, and also of the Betulaceae and Juglandaceae, have long 

 intervals. But, in general, in angiosperms the interval is to be 

 reckoned in days or hours, rather than in the weeks or months of the 

 gjnnnosperms. 



