298 THE ANATOMY OF WOODY PLANTS 



and in the vegetative axes does not manifest the presence of 

 cryptogamic or centripetal wood. In certain genera of the Cyca- 

 dales, both living and extinct, the woody cylinder often shows a 

 curious reduplication, resulting from the appearance of new circles 

 of fibro vascular tissue in the pericycle. The explanation of these 

 structures is somewhat difficult. One view is that they represent 

 a persistence of the numerous cycles of fibrovascular strands in 

 certain types of Medulloseae. This view, however, is rendered 

 difficult by a situation which often presents itself in the Cycadales. 

 In the region of the stem where a cone is attached the medulla is 

 frequently occupied by numerous strands which are absent else- 

 where. It is natural to regard such structures as representing a 

 return of the medullosan organization in relation to the attachment 

 of conservative reproductive axes. If this explanation be adopted 

 for the medullary bundles, the interpretation of the supernumer- 

 ary zones of fibrovascular tissue formed successively in the peri- 

 cycle of the original cylinder cannot be accepted. A more probable 

 elucidation of the situation is furnished by the habit of certain 

 Mesozoic representatives of the group which are contrasted to 

 modern forms by their slender and freely branching habit. This 

 condition, for example, is found in the genus Anomozamites. It is 

 not improbable that the genus mentioned was a climbing plant. 

 The formation of supernumerary zones of fibrovascular tissue is 

 a common feature of organization of the stem in climbing plants 

 of widely diverse gymnospermous and dicotyledonous affinities. 

 It is consequently not unlikely that the Mesozoic forebears of living 

 Cycadales developed successive zones of fibrovascular tissues in 

 their stems as a result of a climbing habit. After this feature had 

 become thoroughly fixed by the lapse of long geological time, a 

 desert habit was superimposed on it with a resulting anatomical 

 organization such as is found in the living representatives of the 

 group. A similar hypothesis has to be invoked, as will be shown 

 in a subsequent chapter, to explain the remarkable anatomical 

 resemblance between the desert-inhabiting gnetalian genus Welwit- 

 schia and the vinelike Gnetum. 



Although there is no instance of the presence of centripetal 

 or cryptogamic wood in the vegetative axis of the Cycadales, 



