3 88 



THE ANATOMY OF WOODY PLANTS 



foliar traces. As a direct result of this discontinuity of the primary 

 cylinder, the secondary cylinder becomes united only by the 

 gradual broadening of the originally separate secondary segments. 

 In c is portrayed the condition found in the stem of a third rep- 

 resentative of the Lycopsida namely, a calamite. Here the 

 primary wood is extremely degenerate, and the secondary segments 

 as a consequence are very narrow and quite widely separated. 



In the herbaceous living 

 Equisetum, the stem of 

 which is depicted on an 

 earlier page, secondary 

 growth has disappeared, 

 being present only as a 

 vestige in conservative 

 organs and regions. 



The series indicated 

 above illustrates the origin 

 of the herbaceous type by 

 degeneracy. The Lycop- 

 sida have been chosen 

 because they reveal the 

 situation in the clearest 

 manner and with the 

 fewest complications. In 

 the Pteropsida the topog- 

 raphy is rendered less easy 

 of comprehension by the presence of the often numerous foliar 

 gaps which characterize the anatomical organization of the 

 siphonostelic central cylinder of that phylum of vascular plants. 

 The principles involved are, however, the same. It may accord- 

 ingly be stated that discontinuity in the primary siphonostelic 

 central cylinder is due either to gaps related to the exits of traces 

 supplying the appendages or to the local degeneracy of the cylinder 

 itself. These interruptions are perpetuated in the early organiza- 

 tion of the secondary cylinder. If the secondary tissues are also 

 degenerate, as in the existing survivors of cryptogamic groups, a 

 pronounced herbaceous condition is the result. This is, however, 



b C 



FIG. 266. Diagram showing the effect of 

 degeneracy of the primary wood on the develop- 

 ment of the secondary cylinder. Explanation in 

 the text. 



