CHAPTER XXVIII 

 THE HERBACEOUS DICOTYLEDONS 



The herbaceous type in the dicotyledons, although not of 

 any definite systematic value, has a considerable significance 

 from the anatomical and evolutionary standpoints. It has been 

 pointed out in earlier pages that discontinuity of the woody cylinder 

 in siphonostelic forms simulating that found in herbaceous dicoty- 

 ledons is frequently present in older groups. In such cases, how- 

 ever, the situation has a very different anatomical explanation 

 and a diverse evolutionary interpretation from that presented 

 by the dicotyledonous angiosperms. In many of the lower groups 

 of plants belonging to both Lycopsida and Pteropsida we find the 

 surviving representatives distinguished from those characteristic 

 of the period of greatest luxuriance by a herbaceous, as opposed 

 to an arboreal, habit. Such herbaceous types owe their origin 

 to degenerative changes and present a distinct contrast to the 

 situation exemplified in the herbaceous dicotyledons, where the 

 modification is the result of differentiation and specialization and 

 not the consequence of mere degeneracy. 



It is necessary to illustrate clearly the mode of appearance of 

 the herbaceous type in the older groups before passing on to the 

 consideration of the situation manifest in the herbaceous angio- 

 sperms. Fig. 2660 illustrates the organization of a siphonostelic 

 stem in a lepidodendrid. Here the primary wood is well developed 

 and constitutes a continuous cylinder. The secondary xylem, 

 which is the result of cambial activity on the inside of the primary 

 phloem, forms a continuous layer interrupted only in the region 

 of exit of a strand destined to a branch, which causes a gap in the 

 primary cylinder that is naturally perpetuated in the earlier 

 organization of the secondary cylinder. In b is shown the condi- 

 tion presented by a sigillarian of Permian age. In this instance 

 the siphonostelic primary cylinder has become degenerate and 

 consequently is only well developed in the regions facing departing 



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