124 THE VASCULAR SYSTEM OF THE AXIS [CH. 



other respects, makes it appear probable that the latter have retained the 

 primitive protostelic structure in relation to their delicate hygrophilous 

 habit. There is no need in such cases to assume any reduction from an 

 ancestry previously more elaborate in anatomical structure. They appear 

 rather as illustrating phyletic inertia — retention of the primitive structure of 

 their ancestors not only in the young, but also in the adult state. On the 

 other hand, the very minute stems of Azolla and Salvinia may well have 

 been simplified in accordance with their aquatic habit from some original 

 protostele of larger size. 



Medullation 



A simple protostelic structure may suffice for small organisms, or for 

 larger ones under peculiar circumstances of growth ; but the great majority 

 of Ferns starting from protostely show in their stelar structure phases of 

 advance, in accordance with the increasing demands made by the more 

 numerous and larger leaves of the growing plant. The same general problem 

 associated with increasing size will have to be met in the ontogeny of any 

 enlarging Fern-Plant. But there is no reason for assuming that the solution 

 need always have been worked out in the same way. The observer ought to be 

 prepared to see modifications of the stele in any of the several phyla, which 

 while tending to the solution of the physiological problem, may differ in the 

 details of execution. The most direct solution is clearly an increase in size 

 of the protostele, and more particularly of its xylem-core. This habitually 

 brings with it an internal differentiation of the stele, and frequently the first 

 step is the appearance of parenchymatous cells, either scattered through the 

 xylem, or constituting a definite medulla or pith in its centre. Evidence of 

 the origin of a pith may be traced in the individual ontogeny. It is further 

 illustrated by comparison of adult living forms, and by study of the strati- 

 graphical sequence of fossils. 



In many small stems, like those of young plants oi Anemia or Schizaea, 

 there is at the very base a small stele with a solid core of tracheides without 

 parenchyma (Fig. ii8, i, ii). Sooner or later as the upward series of trans- 

 verse sections is passed in review (and the point may vary in individuals of 

 the same species) a mass of parenchymatous cells is found to occupy the 

 centre of the xylem. Often it is completely enclosed by a ring of tracheides 

 between the points where the leaf-traces are given off: the condition may 

 then be described as soleno-xylic (Fig. ii8, iii, vi). Where the xylem-ring 

 is thin a sector of this ring becomes detached at the departure of each leaf- 

 trace ; as the sector passes outwards, a parenchymatous connection is 

 established between the pericycle and the central mass of parenchyma (Fig. 

 1 1 8, iv, v). This central parenchyma may in the ordinary sense of the term 

 be called a "pith," or "medulla." In such cases as Anemia and Schizaea, as 



