316 GYMNOSPERMS 



in the Zurich material the interval was over 2 months. In Torrey 

 laxifolia, pollen is shed late in March or early in April, and fertiliza- 

 tion, in 1904 material, took place August 12. 



In this rather extensive survey, there are slight differences in the 

 dates given by different observers. Most of the dates are for ma- 

 terial growing in its native habitat, but some are from exotic ma- 

 terial. However, this factor is not likely to influence the times of 

 pollination and fertilization as much as would the difi'erences in 

 latitude, altitude, and immediate surroundings. While these factors 

 certainly influence the time of pollination, the interval between 

 pollination and fertilization does not seem to be affected. 



In conifers with a long period between pollination and fertiliza- 

 tion, there is usually a rapid growth of the pollen tube immediately 

 after pollination. The tube grows down about to the level of the free 

 part of the nucellus, and then stops and winters in this condition, 

 resuming its growth the next spring. In forms with two growth- 

 periods, and in which the division into stalk and body cell does not 

 take place before the pollen is shed, the division into stalk, and body 

 cell occurs during the first period. The division of the body cell to 

 form the two sperms takes place shortly before fertilization, usually 

 less than a week before the fusion of gametes. In several closely 

 observed forms, the interval is about 5 days. 



In almost everything there are exceptions. In Abies balsamea 

 the body cell divides before the tube begins to form; but here the 

 growth of the pollen tube through the nucellus to the egg does not 

 take more than 2 days, and the time may be reckoned in hours. Con- 

 sequently, the interval between the division of the body cell and 

 fertilization is not so long as in cases where the division takes place 

 late in the development of the pollen tube. 



The sperms. — The sperms, or male gametes, of conifers show con- 

 siderable differences in organization, but there are only two cate- 

 gories : the sperm is either a highly organized cell, or it has lost its cell 

 wall, and, at the time of fertilization, its cytoplasm may have blend- 

 ed more or less with that of the pollen tube. 



The most primitive form is, doubtless, the highly organized cell, 

 as it is found in Juniperus and other members of the Cupressaceae, 

 Taxodiaceac, Podocarpaceae, and Taxaceae (figs. 315, 316). These 



