THE SPERMATOPHYTA : GENERAL INTRODUCTION 655 



accept the idea that all parts of a plant are differentiations from a common 

 structure, a shoot or telome, and the antitheses of the theories are perceived 

 to be false. 



The objection urged against all phytonic or segmentation views of the 

 axis, that they necessarily imply a vascular system which is common to stem 

 and leaf and preclude the existence of a true cauline stele, is no longer a 

 difficulty if we accept it as a pointer to a necessary limitation of such theories, 

 namely, that they should be applied only to the Spermatophyta, which alone 

 possess such a common vascular system. The Pteridophyta for the most 

 part show the most definite anatomical evidence of the continuity and unity 

 of the stem as an independent axis, which probably represents a direct 

 derivation from a primordial filament, or protaxis, though we may perhaps 

 allow for a peripheral cortication in Potonie's sense, at least in the 

 megaphyllous types. A significant exception is found in the Marattiaceae, 

 in which megaphylly is so pronounced that the stem has become com- 

 paratively insignificant and has - a vascular system which agrees with that 

 of the Spermatophyta in being of foliar origin. 



This may be a clue to the nature of the change of vascular structure 

 which has occurred between Pteridophyta and Spermatophyta. In protaxial 

 structures the axis predominates and the vascular supply to the leaves is 

 dependent on the size of the stem, not on the actual requirements of the 

 leaves themselves. This is ill-adapted, physiologically, to produce a satis- 

 factory balance between supply and demand in the flow of materials to the 

 leaf, a maladjustment which would become more pronounced with the 

 greater extension of the leaves, with the development of the tree habit, and 

 perhaps most of all with the production of leaf-borne seeds, which would 

 make large periodic drafts on the conduction of food reserves to the leaves. 

 The extreme complexity of the stele and the extraordinary' development of 

 adventitious roots in tree-like Ferns, both living and fossil, serve to illustrate 

 the difficulties inherent in the protaxial type of plant architecture. The 

 Marattiaceae, on the other hand, show us a way out of the impasse, namely, 

 the contraction of the stem to such a small size that the leaf traces become 

 directly linked to each other and to the vascular system of the roots, forming 

 a common system like that of the Spermatophyta and eliminating the necessity 

 for a cauline stele. Once established, this newer system, which allowed 

 each leaf direct access to the absorptive system, would surely prove more 

 elastic and expansible in the direction of tree-growth than the old. The 

 axis, rebuilt on a vascular skeleton of longitudinally elongated leaf traces, 

 would, however, be no longer a simple structure, but a pseudaxis, as Delpino 

 called it, and such, in the Spermatophyta, it seems to be. One may perhaps 

 go further and suggest that the elimination of centripetal wood, which is 

 known to have taken place during the evolution of the seed plants, accom- 

 panied the disappearance of the protaxis or at least the redistribution of its 

 tissues among the component leaf units of the pseudaxis. 



No doubt a great part of the theoretical morphology which we have 

 discussed may be criticised as mere speculation, but we would not therefore, 



