CHAPTER XVIII 



THE GYMNOSPERMAE : CYC AD ALES, GINKGO ALES 



AND GNETALES 



Cycadales 



The order Cycadales includes one family, the Cycadaceae, which is a small 

 group of genera distributed over the warmer parts of the globe, though 

 nowhere very abundant. They are of small account in the vegetation of the 

 present day, but they are of great botanical interest as the remnant of a group 

 which reached its zenith in the Cretaceous period and which can be traced 

 back throughout the Mesozoic era. Many features in their organization 

 are more primitive than anything found in the Coniferales, and they show 

 evidence of a fairly close connection with the great Palaeozoic group of the 

 Pteridospermae, from which most of the modern Flowering Plants are 

 probably derived. 



Morphologically they occupy an intermediate position between the 

 Pteridosperms and the Angiosperms, and although none of the living Cycads 

 can be considered to be a direct ancestor of the Angiosperms, there is a strong 

 probability that the latter group sprang from an earlier Cycadean stock. 



The nine living genera are all of somewhat similar habit, having usually 

 short, thick stems crow-ned by a cluster of very large pinnate leaves, so that 

 they resemble in aspect the Tree Ferns or some Palms. Their growth is very 

 slow and branching is rare. 



The principal difference between the Cycads and the Pteridosperms is 

 that in the Cycads the sporophylls are organized into distinct strobili or cones, 

 which are borne at the apex of the stem or, in some cases, laterally, but near 

 the apex. They are verv much larger than the cones of the Pines, being 

 sometimes as much as 3 ft. long and 9 in. thick. Unlike the Conifers, the 

 Cycads are all dioecious. 



The discontinuous geographical distribution and the small areas occupied 

 by most of the individual genera are characteristic of old and decadent groups. 

 In the middle part of the Mesozoic era the Cycads seem to have been so much 

 more abundant and widespread than at the present day that this geological 

 period has sometimes been called the " Age of Cycads," but this is not 

 strictly correct, since the dominant plants of that period were the similar 

 but hermaphrodite Bennettitales with which their fossil remains were 

 formerly confused. 



The following are the leading characteristics of the living genera : 



Cycos. The most primitive genus, with sporophylls of primitive leaf- 

 like form, which suggest a close connection with the Pteridospermae. The 

 genus occurs in Africa, Australia and East Asia. 



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