132 THE VASCULAR SYSTEM OF THE AXIS [CH. 



nected with the outer endodermis. In a small plant it was found that the inner endodermis 

 was not constantly present (Fig. 125). As the stele was followed forward acropetally from 

 the broken base, it was found that the pith and inner endodermis decreased until the latter 

 was narrowed to a vanishing point, and the simple medullated protostele was the result. 

 Later the pith expanded and a tubular inner endodermis again appeared. It subsequently 

 narrowed again to a vanishing point, the pith at the same time being reduced. Thus for 

 a second time a simple medullated protostele resulted. On subsequent re-expansion of 

 the pith an inner endodermis again appeared and was continued onwards to the apex. 

 A probable explanation of these changes is that the formation of an inner endodermis is 

 causally related to the expansion of the pith which is often sclerosed in xerophytic Ferns. 

 Its reduction, or even its absence from those zones where the xylem is protostelic, accom- 

 panies the reduction of the pith. It is suggested that here the function of the inner endo- 

 dermis is to establish a further intra-stelar physiological control over the water-storage-pith 

 where it is bulky. But this is unnecessary where the pith is small. The whole may thus 

 be regarded as an upgrade development, involving a formation of an inner endodermis 

 de novo. Unfortunately the ontogenetic development is not known in this plant. 



It has been seen in the juvenile stem oi Anemia^ and in the relatively simple Schizaea 

 rupestris^ how medullation is initiated in the protostele. The latter appears to have per- 

 manently retained this state of simple medullation. But other species of Schizaea show 



Fig. 125. Reconstruction from sections of the stele of a small plant oi Platyzoma. The base is to the 

 left. Outer and inner endodermis are represented by black lines : phloem is hatched, and xylem is 

 black. Pericycle, inner parenchyma, and pith are white. The diagram represents the general 

 arrangement of tissues as would be seen in a median longitudinal section of the stele. (After Dr 

 M'=Lean Thompson.) (xio.) 



further intra-stelar complications. For instance in the adult state of the large S. dichotoma 

 the pith is sclerotic, but always with thin-walled parenchyma lining the xylem-ring. Within 

 this thin-walled parenchyma isolated endodermal spindles may arise, quite independent 

 of the outer endodermis, and with no obvious relation to the leaf-insertions (Boodle, Aiui. 

 of Bot. XV, p. 703; Thompson, Trans. Roy. Soc. Edin. lii, 1920, p. 718). The matter is 

 still further complicated by the observation of groups of tracheides either lodged within 

 the spindles, or in the parenchymatous pith outside them. Such observations suggest a 

 high variability of determination of the cells of the procambium. 



Schizaea inalaccatta appears to be a smaller example of structure substantially of the 

 same type (Tansley and Chick, Ann. 0/ Bot. xvii, p. 493). 



The origin of the pith in the Osmundaceae has already been accounted 

 for (p. 128). But the large upright stocks of this family present further com- 

 plications which are intra-stelar in their nature. It has been noted how in 

 Osmwidites Kolbei the outer xylem-ring, which is continuous in TJiamno- 

 pteris (Fig. 123), is interrupted at the departure of each leaf-trace. This is the 

 rule in the adult stems of all living Osmundaceae, though not in the juvenile 

 plants. The ontogeny of Osumnda, which was followed by Gwynne-Vaughan 



