366 



GENERAL MORPHOLOGY OF PLANTS 



Fig. 33 o. 





on the underside of the prothallium, from the thicker or cushion 



portion, since they are larger than the sperm cases and need 



more plant food for their own growth, 

 and for the young fern embryo. The 

 egg case is flask- shaped, the venter 

 sunk in the tissue of the prothallium, 

 while the neck projects beyond, and 

 in our common ferns (PolypodiacecB) 

 curves slightly backward toward the 

 small end of the prothallium. When 

 - the spores are crowded and the light 

 is very weak, so that they get but little 



nutriment, they often produce only protonemal threads which 



bear only sperm cases. On the normal prothallia, sperm cases 



are borne only on the young prothallium, while the egg cases 



are borne later on the older tissue. In this way cross fer- 



tilization is usually brought about 



between two different prothallia. 

 520. Fertilization. A t the 



time the egg case is mature, the 



cells in the canal of the neck 



dissolve into a gelatinous sub- 



stance, which oozes out at the 



opening, leaving a canal down to 



the egg in the venter. The sperm 



case is ruptured by absorbing 



water, as after a rain, or in green- 



houses when they are watered. 



This absorption of water pro- 



duces such a pressure that the 



terminal cell is broken, and the 



sperms are shot out. As they swim 



around in the water some come in the vicinity of an open egg case 



(usually of another prothallium), are more or less entangled in 



the slime, and make their way down to the egg. One sperm enters 



and unites with the nucleus of the egg and completes fertilization. 



Fig. 331- 



Archegonium of fern. Large cell in 

 the venter is the egg, next is the ventral 

 canal cell, and in the canal of the neck 

 are two nuclei of the canal cell. 



