THE LEAF 



201 



divisions of the leaf. The cryptogamic wood is here relatively bet- 

 ter differentiated than in the axis of the leaf, and it is obvious that 

 the seriation of its elements is toward the upper surface of the foliar 

 organ. The centrifugal wood is very slightly developed. In the 

 next figure (Fig. 148) appears a foliar bundle of Cycas revoluta in its 

 course in the cortex of the stem. There is a mass of irregularly 

 arranged primary wood in the center, surrounded by the radially 

 disposed second- 

 ary wood, which in 

 turn is followed by 

 the tissues of the 

 phloem. The phloem 

 entirely surrounds 

 the bundle, and this 

 is consequently of 

 the concentric type 

 often found in the 

 living ferns and their 

 allies. The contrast 

 in organization of 

 the bundles of the 

 leaf in different 

 parts is very strik- 

 ing. The lower con- 

 centric region, in 

 accordance with 



generally accepted principles of comparative anatomy to be eluci- 

 dated later in a special chapter, may be regarded as supplying 

 evidence of the former concentric character of the woody cylin- 

 der of the stem in the Cycadales, a hypothesis entirely justified 

 by the existence of a remarkable group of fernlike seed plants 

 in the Paleozoic, known as the Medulloseae, from which on both 

 anatomical and reproductive evidence the living group seems to 

 have been derived. In these forms the bundles of the stem were, 

 as will be subsequently shown, often concentric in their organiza- 

 tion. In the higher region of the leaf trace in the cycads the 

 concentric character is lost, but the centripetal development of 



FIG. 148. Concentric foliar bundle from the cortex 

 of Cycas revoluta. 



