. Botany of the living plant. Botany; Plants. THE RELATION OF SIZE AND FORM IN PLANTS 599 distribution of nutrients to the growing region from below. But structurally we see in many Ferns how distribution of nutrients may be helped by accessory strands which traverse the column of the pith, either in the form of a single protostele, a solenostele, or a concentric group of them {Psaronins) : or again there may be a number of separate medullary strands. In point of fact, there is considerable variety in these medullary systems. But they are all primary, differing fundamentally from the secondary
. Botany of the living plant. Botany; Plants. THE RELATION OF SIZE AND FORM IN PLANTS 599 distribution of nutrients to the growing region from below. But structurally we see in many Ferns how distribution of nutrients may be helped by accessory strands which traverse the column of the pith, either in the form of a single protostele, a solenostele, or a concentric group of them {Psaronins) : or again there may be a number of separate medullary strands. In point of fact, there is considerable variety in these medullary systems. But they are all primary, differing fundamentally from the secondary growths of cambial origin, which are characteristic of " exogenous " Fig. 454. Nephrolepis cordifolia. A = stolon bearing a tuber, in which the protostele breaks up into a cylindrical network, contracting again at the apex. R =root (after Sahni). B = transverse section of a protostelic stolon ( x 5). C = transverse section of tuber, showing ring of meristeles each limited by endodermis. Diameter of stolon, i-6 mm. Diameter of tuber, n mm. (From Size and Form, p. 132.) The functional analogy between such medullary systems as those of Ferns and the vascular strands scattered through the pith of the expanded stele of Palms, Maize, or Sugar Cane, appears obvious, though their morphological origin is not the same : nor is their physiological effectiveness identical. Nevertheless, both follow on stelar expansion. Both types might be included under the old term "' endogenous " : but since the plants in which they appear have no near affinity, and arise along quite distinct evolutionary trends, their comparison cannot be held as more than one of analogy. Whether small or great the apical bud acts as a physiological unit, and the stelar system serves it as a common unit of supply from below to the apical region. In the more advanced Ferns the conducting tissues are sheathed by endodermis, which by the nature of its cell-walls acts when mature as a physiolog
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