90 THE LIVING CYCADS 



In the Paleozoic ancestors of the cycads the ovules 

 were borne on leaves with little or no modification from 

 the vegetative type, and from this condition there has 

 been a gradual reduction from sporophylls closely re- 

 sembling foliage leaves to those so profoundly modified 

 that such resemblance is nearly obliterated. If the 

 living cycads could be arranged in a genetic line, on the 

 basis of this single character, the Hne would begin with 

 Cycas and end with Zamia. 



The sporophyll in Cycas bears several ovules, but 

 in all the other genera there are regularly just two ovules, 

 one on each side of the stalk. 



It is impossible to understand the ovule, which in its 

 later development is called the seed, without noting a 

 couple of stages in its fern ancestry, for the ovule is the 

 lineal descendant of the sporangium, or ''spore case," 

 of the ferns. The sporangia of a common fern, like the 

 "Boston fern," contain many spores, all of which look 

 ah'ke and at maturity fall upon moist soil, where they 

 give rise to green "prothallia," which finally give rise to 

 structures bearing eggs and sperms (Figs. 33-36). 



In Selaginella, one of the fern allies which can be seen 

 in any greenhouse, the development starts just as in the 

 common fern, but only four of the prospective spores 

 develop, the others becoming abortive and serving as 

 nutritive material for the four, which become so large 

 that they are called "megaspores." When they germi- 

 nate, the prothallium does not come out and become 

 green but remains within the mcgasporc, and the niega- 

 sporc itself may remain within the sporangium until 

 the eggs are produced, or even longer (Figs. 37 and 38). 

 In the common fern the prothallium usually produces 



