CHAPTER XVI 



GYMNOSPERMS 

 CYCADALES (CYCADS) 



A knowledge of the life history of the cycads will be of special 

 interest and importance to the student at this point, since botan- 

 ists have conclusively shown in recent years that they are inter- 

 mediates between the highest spore plants, represented by the 

 heterosporous ferns and selaginellas, and the higher seed plants, 

 like the pine and the bean. 



Some botanists believe that the flowering plants arose from 

 cycadlike ancestors in an earlier geologic period of the earth's 

 history, while all are agreed that the modern cycads resemble 

 closely a- great plexus of plant groups which in Carboniferous 

 times bridged the gap between the highest living heterosporous 

 pteridophytes (namely, Selaginella) and such seed plants as the 

 pines and the flowering plants. The intermediate character- 

 istics of the cycads have been shown to involve not only the 

 reproductive features in their life history but also their general 

 habit and vegetative structure. 



ZAMIA 



SPOEOPHYTE 



In Zamia (Fig. 187) the leaves resemble those of ferns in 

 size and form, and the tuberous stem is covered with the scale- 

 like bases of former leaves, as in many tree ferns. The internal 

 anatomy of the above organs, as well as their external form, indi- 

 cates also a fernlike ancestry. The male and female strobili are 

 borne on separate plants and resemble in form and size the cones 

 of pines. The microsporophylls are much larger, however, than 



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