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 " plants. 



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 

 physiological barrier (Figs. 377, 378). But its control is not complete, being 

 subject to progressive phases of development. In its adult state it may limit 

 gaseous interchange, and serve as an efficient boundary in guiding supplies 

 of nutrients in solution upwards. But as the younger tissues of the growing 



