Gymnosperms : The Cycads 507 



In the cycads, then, the male gametophyte is a pollen tube. 

 As in the flowering plants, it is parasitic. It is unique, however, 

 in its manner of growth and in producing motile sperms — a 

 habit that has been carried along through all the Bryophytes 

 and Pteridophytes from the swimming sperms of the algae. 



The female gametophyte. A single megaspore is formed 

 within each ovule or megasporangium. The megaspore germi- 

 nates inside the ovule, and utilizing the food in the inner soft 

 tissue (nucellus) of the ovule ultimately fills most of the space 

 inside the hard wall of the ovule. This is the female gameto- 

 phyte. Like the female gametophyte of Selaginella, it develops 

 within the megasporangium, but in this case the gametophyte is 

 entirely shut away from the light. Like the pollen tube, it is 

 wholly parasitic. 



The female gametophyte at maturity organizes several arche- 

 gonia that consist merely of two small neck cells and the very 

 large egg cell. The neck cells open into the pollen chamber at 

 the time of the liberation of the sperms from the pollen tube. 



Fertilization. A sperm moves down between the neck cells 

 and enters the egg. The sperm nucleus slips out of its covering 

 of cytoplasm and cilia and unites with the egg nucleus. The 

 fertilized egg is the first cell of the new sporophyte generation. 

 It soon begins to divide and ultimately forms the embryo. The 

 embryo pushes back into the gametophyte tissue until it occupies 

 the whole longitudinal axis of the seed. The embryo has two 

 cotyledons and develops the growing points of the stem and root. 



The seed. The seed at maturity has an outer soft fleshy 

 layer surrounding a stony layer. Within these seed coats is a 

 membranous tissue, the remains of the nucellus and inner fleshy 

 wall of the sporangium. Next inside is food-containing tissue 

 (usually termed endosperm), the remnant of the female ga- 

 metophyte. The gametophyte incloses the embryo or young 

 sporophyte. 



REFERENCE 



Chamberlain, C. J. The Living Cycads. University of Chicago Press, Chicago 



