COMPARATIVE MORPHOLOGY OF GYMNOSPERMS 155 



Ephedra, if not in the other Gnetales, while the very regular 

 massing of these tracheids to form the secondary wood is charac- 

 teristic of Conifers. 



It is also worthy of note that there are no distinct nodes in 

 the stems of Cycadales and Bennettitales ; that they are usually 

 evident, but often indistinct in Ginkgoales, Cordaitales, and 

 Conif erales ; and that in Gnetales they are distinctly developed. 



THE LEAF 



The spiral and cyclic arrangements are definitely separated 

 by great groups, the Cupresseae and Gnetales being cyclic, the 

 other Gymnosperms spiral. 



The tendency to heterophylly is a very strong one, scale 

 leaves being associated with foliage leaves in every great group. 

 In Cycadales there is a regular alternation of the two types, 

 while in Coniferales the extreme of diversity is reached. In 

 Coniferales the primitive type of shoot seems to have consisted 

 of axes all of which were clothed with free needle leaves, which 

 later became intermixed with scales ; while in the highly spe- 

 cialized genus Pinus the foliage leaves abandon the main axes, 

 and are pushed out on dwarf shoots to the extremities of the 

 branch system. 



The foliage leaves are remarkably varied in form and size 

 and venation, being pinnately branched and more or less dichot- 

 omously veined in Cycadales and Bennettitales, broad and dichot- 

 omously veined in Ginkgoales, characteristically acicular in most 

 Coniferales, ribbonlike and parallel-veined in Cordaitales and 

 Tumboa, and broad and reticulately veined in Gnetum. It is 

 also a noteworthy fact that the fernlike leaves of some of the 

 Cycads also show the characteristic circinate vernation of Ferns. 



Although variable in form and venation, the foliage leaves 

 of Gymnosperms are fairly constant in certain points of struc- 

 ture, all of them, with few exceptions, developing rigid pro- 

 tective structures, and having a limited distribution of the vas- 

 cular system, with a characteristic development of " transfusion 

 tissue." 



THE EOOT 



So far as studied, the roots of Gymnosperms seem to be of 

 very uniform structure. As distinct from Pteridophytes the 



