334 SCIENCE PROGRESS 



(2) The majority of vascular arrangements are referable to the 

 siphonostele. 



(3) The pith is extra stelar, i.e. it is part of the ground or 

 fundamental tissue, and has no separate morphological value. 



Boodle, who has contributed largely to our knowledge of the 

 anatomy of the Ferns, is opposed to the views just enumerated. 

 The main points in his position are as follows : 



(1) The endodermis is not a dependable morphological limit. 



(2) The medullated monostele and the solenostelic (and 

 dialystelic) types have been derived from the protostele by the 

 transformation of its central tissue into parenchyma, and into 

 parenchyma together with phloem and endodermis respectively. 



(3) The pith is thus stelar in origin. 



The differences of opinion between these two observers are 

 mainly concerned with the morphology of the endodermis and 

 pith ; and, indeed, it is around these two points that controversy 

 centres. The majority agree as to the facts ; it is in the inter- 

 pretation of the facts that disagreement exists. 



The Morphological Value of the Endodermis 

 It has been seen, from the work of Schoute and others, 

 that, to the minds of many, considerable difficulties lie in the 

 way of accepting the endodermis as a morphological layer — 

 or as a layer which may be relied upon as delimiting the 

 boundary of the cortex — owing to the fact that in many cases 

 it is not the innermost layer of the cortex of Angiosperms. 



In the case of the polystelic, or, better still, dictyostelic 1 

 plants, the apical region contains a number of strands, made 

 up of many immature and dividing elements, showing no 

 differentiation into definite tissues, and directly continuous with 

 the fully developed steles. Taking the tissue between these 

 immature and developing strands, where is the line of demar- 

 cation between periblem and plerome ? Then again, the 

 endodermis exhibits much variation in its appearance and 

 non-appearance, in places where, theoretically, it should be 

 absent and present respectively. 



Lewis (43) found a well-marked endodermis enclosing a mass 

 of cortical parenchyma in a root of Ruscus. In Helminthostachys 



1 A purely descriptive term suggested by Brebner (8) to replace the term 

 polystele in order to avoid the theoretical connotation inseparable from the 

 latter. He defines the dictyostele as "a vascular tube with large overlapping 

 leaf- gaps, so that the whole structure becomes a network of vascular strands." 



