FERTILIZATION 117 



The amount of liquid discharged by a single pollen tube 

 is small in comparison with the size of the archegonial 

 chamber, and if the liquid should spread evenly it would 

 not be sufficient to cover the sperm ; however, it behaves 

 somewhat like a drop of water on a greasy surface, not 

 spreading much, but moving until it comes into contact 

 with the neck of an archegonium. 



What causes the sperm to enter the egg? In ferns 

 it has been shown that the sperms are strongly attracted 

 by certain chemical substances in the neck of the 

 archegonium and are thus drawn to the egg. My own 

 experiments and those of the Japanese botanist Miyake 

 prove that there is no such chemical attraction in the 

 cycads, even the material of the egg failing to exert any 

 stimulus. 



In Dioon, Stangeria, and other cycads, just before 

 fertilization, numerous preparations show a little proto- 

 plasm about the necks of the archegonia, and for a long 

 time I assumed that it had been squeezed out of the egg 

 by the pressure of the knife, as a square piece containing 

 the archegonia was being cut out from the top of the 

 endosperm for the purpose of making thin sections for 

 detailed microscopic study. It was also noted that 

 when material near the fertilization period is dropped 

 into water or into some preservative a small bubble 

 appears at the neck of the archegonium. 



These phenomena suggested an explanation of the 

 presence of the protoplasm about the necks of the arche- 

 gonia, and also suggested how the sperm might get into 

 the egg. 



The drop of liquid discharged from the pollen tube 

 has a very high osmotic pressure, and when it comes 



