144 ELEMENTARY STUDIES IN BOTANY 



of pollen by the way. This landing place of pollen in An- 

 giosperms is very different from that in Gymnosperms. In 

 the latter group the pollen reaches the tip of the nucellus, 

 but in the former group it can reach only the surface of the 

 carpel that incloses the ovules. This means that in Gym- 

 nosperms the male cells (in the pollen grain) are separated 

 from the egg only by the tissue at the tip of the nucellus, 

 while in Angiosperms the male cells are separated from the 

 egg not only by the tissue at the tip of the nucellus, but also 

 by the style and the ovary cavity. 



After pollination has been accomplished, therefore, there 

 must be an extensive development of the pollen tube before 

 fertilization can be accomplished (Fig. 115). A good pollen 

 grain lying on the stigmatic surface begins to send a tube into 

 the style, and into the tip of the tube the two sperms pass. 

 The growth of the tube is started by a sugary secretion of 

 the stigmatic surface ; it continues its growth down through 

 the style by means of food material supplied by the adjacent 

 cells, enters the ovary cavity, and grows along its wall, enters 

 a favorably placed micropyle, reaches the tip of the nucellus, 

 grows on through the tip of the nucellus, pierces the wall of 

 the embryo-sac, and discharges its two sperms, which at last 

 have free access to the egg. 



Before fertilization is possible, therefore, there must be 

 pollination and the growth of a pollen tube. It is strange 

 that pollination and fertilization should ever be confused, 

 when they are separated by such an extensive performance 

 as the growth of the pollen tube. In some Angiosperms 

 (and in many Gymnosperms) fertilization does not occur 

 until a year after pollination, and the time interval varies 

 in different plants between a year and a few hours. 



A remarkable situation is developed in Angiosperms in 

 connection with fertilization. Two sperms are discharged 

 into the embryo-sac by the pollen tube, and there is only one 

 egg. For a long time it was thought that one sperm united 



