POLLINATION AND FERTILIZATION OF FLOWERS. 223 



nucellus the cells of the pollen of Gymnosperms behave differ- 

 ently. 



The fertilization of Piniis syhestris, as worked out by Dixon* 

 in Strasburger's laboratory, may serve as an illustration. "In 

 this species about thirteen months intervene between the time of 

 pollination and that of fertilization. "f In the ripe pollen-grains 

 are to be seen a small prothallium cell, (the last formed of the 

 cell group already described), and a large nucleus; the latter 

 passes into the pollen-tube which is formed immediately after 

 pollination. The tubes penetrate a short distance into the hard 

 brownish tissue at the top of the nucellus, where they remain 

 quiescent till the following spring, presenting the appearance 

 shown in fig. i. Late in April the pollen-tube becomes filled 

 with starch, and the prothallium cell divides, as already noted, 

 into a small stalk-cell ; the latter soon separating into two cells 

 of about ecjual size, the male sexual cells. About the same time 

 the wall of the stalk-cell is ruptured, and its nucleus follows the 

 two sexual cells into the pollen-tube. 



The growth of the tubes through the brown tissue at the top 

 of the nucellus is verv slow, and at this time the tube often 

 branches two or three times, as shown in the cut ; eventually, 

 however, but one branch is continued. The cells of this upper 

 part of the nucellus are relatively poor in starch contents, but 

 the tube is often gorged with starch and the action of the 

 enzymes is evident in the way in which the tubes penetrate the 

 neighboring cells, filling them with a brownish substance. 



After leaving the brownish portion of the nucellus, the growth 

 of the tube through the thin walled tissue is comparatively very 

 rapid ; the nuclei of the pollen-tube and of the stalk-cell begin to 

 degenerate, and in about ten days — about a month from the time 

 active growth of the tube commenced — the embryo sac was 

 found to have been reached, while in many cases, fertilization 

 had occurred. "When the pollen tube reaches the oosphere," 

 says Dixon,! "not only do the sexual nuclei pass into the latter, 



*Dixon, H. H.; "Fertilization of Piims sylvestris," Ann. of Bot., VIII, 24, (1S04). 



tTbis statement is made by Dixon, and is also given bj' Strasburger in several 

 of his works (See Practical Botanj', p. 306, 2a Eng. Ed. by Hillliouse), but in the 

 work here detailed it would seem that the time is nearer 12 than 13 months. The 

 cones are said to be receptive about the end of May, and as shown by Dixon, fer- 

 tilization takes place about the same season of the year. 



tl. c, 27. 



