198 THE ANATOMY OF WOODY PLANTS 



The next important modification in the case of the fibrovascular 

 structures of the stem is the appearance of aggregations of rays by 

 reason of the increased efficiency of the secondary fibrovascular 

 cylinder in regard both to its conducting and to its storage elements. 

 The clusters or congeries of rays originally diffused throughout 

 the woody structure tend in higher groups to become somewhat 

 definitely related to the appendages. By further progress in the 

 evolution of radial parenchyma we have the compound and diffuse 

 types of rays making their appearance, the former particularly 

 characteristic of herbaceous types, and the latter of the mass of 

 dicotyledonous forest trees. The appearance of large rays, inter- 

 rupting the continuity of the central cylinder and bringing about 

 the differentiation of separate fibrovascular strands, is a particular 

 feature of the herbaceous type representative of later geologic time 

 and of cooler climatic conditions. 



The woody cylinder broken up into fibrovascular bundles as 

 the result of the evolution of storage devices in connection with the 

 appendages in the herbaceous dicotyledons, in the monocotyledons 

 undergoes further modification in relation to decrease in size and 

 increase in number of the bundles in correlation with the greater 

 efficiency and consequently more numerous conducting strands 

 of the leaves. The decrease in size of the bundles is definitely 

 related to the loss of the cambial activity which characterized the 

 fibrovascular structures of the monocotyledonous angiosperms as 

 a whole. The increase in number of the cauline bundles, resulting 

 from the entry of very numerous leaf traces into the stem at the 

 nodes, brings with it the distribution of the fibrovascular strands 

 throughout the transverse section of the stem. The monocotyle- 

 dons may, in a sense not altogether figurative, be said to represent 

 the second childhood of the vascular plants, just as the Pteridophyta 

 constitute its true infantile phase of development. It is necessary, 

 however, to distinguish very clearly between the primary structures 

 of the monocotyledonous angiosperms which are the result of the 

 loss of secondary growth, and the primary structures of the Paleo- 

 zoic forms which, so far as we know, are a primitive feature of 

 organization. 



