COMMON CYCADS 273 



and increase its thickness by the development of their 

 cambium, and by the formation of new bundles in the 

 cortical meristem. The sporophylls 

 which bear microspores and megaspores 

 form more or less distinct cones (strobili) 

 but occur on separate plants (dioecious). 



489. The common greenhouse Cycad 

 (Cycas revoluta) produces elongated, 

 compact cones of microsporophylls, 20 

 to 30 centimeters long and 5 to 6 centi- 

 meters thick. Each sporophyll bears on 



its lower surface numerous small scattered microspor- 

 angia containing microspores, constituting the so-called 

 "pollen." These microspores fall out, and on germi- 

 nation produce a small one- or two-celled gametophyte, 

 and a tubular antherid containing two spirally many- 

 ciliated sperms (about 0.2 millimeter in diameter). The 

 megasporophylls constitute a loose terminal cone on the 

 main axis of the tree. Each sporophyll bears several 

 laterally placed megasporangia each of which has become 

 covered with an indusium-like structure (integument). 

 Within the body of the sporangium (now known as the 

 ovule) a megaspore develops, but this at maturity does 

 not fall out but remains surrounded by nutrient tissue. 

 Here it germinates and develops a solid, many-celled 

 spheroidal gametophyte, and at its summit forms sev- 

 eral deeply sunken archegones, in which the eggs are of 

 remarkably large size (2 to 3 millimeters). 



490. Fertilization of the egg takes place as follows: 

 The microspore is carried by the wind or other means to 

 the opening (micropyle) at the summit of the ovule 

 integument; there it germinates, the tubular antherid 

 penetrating the adjacent tissues; the sperms escape by the 

 rupture of the tube, and swim through the intervening 



