350 BOTANY: PRINCIPLES AND PROBLEMS 



microsporophyll (Fig. 222) bears many sporangia or anthers and 

 produces a large amount of pollen. The ovules are generally 

 large and thick-walled, and the megaspore, arising as one of four 

 potentially spore-producing cells in the middle of the nucellus, 

 develops into a large embryo-sac. At the tip of the nucellus. 

 just under the micropyle, a large, liquid-filled pollen chamber 

 arises. The pollen grain enters this chamber through the micro- 



FiG. 219. — Cycas revohita, one of the Cycadales. Male plant with cone. (Photo 

 hyG. S. Torrey). 



pyle and there germinates. Its two male cells are each provided 

 with a spiral band of cilia and swim about in the liquid, a remark- 

 able persistence of the habit of swimming sperms which harks 

 back through pteridophytes and bryophytes to their remote 

 algal ancestry. A pollen-tube is formed and penetrates the 

 adjacent nucellar tissue, but it seems rather to absorb food than 

 to assist in the transference of the male cells, for the pollen- 

 chamber gradually enlarges itself until it reaches the embryo- 

 sac, where one of the sperms enters an archegonium and effects 

 fertilization. 



Cycads are confined to the warmer regions of the globe. They 

 are an inconspicuous group today but in earlier ages, notably in 

 the Mesozoic Era, were abundant and diversified. Their 

 close relatives, the Bennettitales (now extinct), were for a time 



