RELATION OF LEAF TO AXIS 



197 



formation of a regular stele: in which case the leaf-traces, represented in 

 the first case by the protoxylem-strands, would be phylogenetically prior : 

 the central metaxylem, on the other hand, would be a later formation, 

 developed in the larger forms to furnish additional conducting channels to 



FIG. 99 



A forked sporangiferous branch of Lycoftndium chamtrcyparissusm longitudinal .section, 

 slightly magnified. jf/= the ,-ixile vascular body. bb = leaves. ss = young sporangia. 



supplement the protoxylems in supplying the needs of the higher foliage 

 leaves and sporophylls. 



In approaching an opinion on such a question much will turn upon tin- 

 initial relation of the leaf to the axis in the evolution of the leafy sporo- 

 phyte, a matter already discussed in Chapter XL If the leaf were from 

 the first a preponderating influence in the shoot, then the latter explanation 



