II.—Size in Relation to Internal Morphology. No. 3. The Vascular System of Roots

Abstract

In recent papers published in the Proceedings and Transactions of the Society the importance of Size as a factor determining the morphology of the vascular system of Pteridophytes has been discussed (2, 3, 29). The purpose of this memoir is to submit data concerning the vascular system of roots, and to determine to what extent Size may have acted as a causal factor in influencing the morphology of these structures. It has been pointed out that the most useful material for these causal studies is to be found where there is only primary vascular tissue, free from complexities such, for example, as the formation of secondary wood. In the case of leafy shoots the vascular system is often profoundly influenced by the insertion of the appendages, especially where the latter are of the megaphyllous type. In many of these instances the resulting vascular structure is due to a complex of so many interacting factors that the specific effect of any one is very difficult to determine. To understand the influence of any one factor in particular we must turn, as Professor Lang has pointed out, to the study of favourable cases (19). These are to be found where the structure is of primary nature, unaffected by the activity of secondary meristems, and where the organ in question is influenced as little as possible by the insertion of appendages.