36 
Lang,—Studies in the Morphotogy and 
juvenile and adult rhizomes, critical consideration shows that even the small 
rhizomes with their central elements spirally thickened are to be regarded 
as mesarch. The mesarchy is shown where the protoxylem for the leaf-trace 
appears (cf. Photo 30). This interpretation is supported by the behaviour 
of the spirally thickened tracheides of the inner xylem at such leaf-trace 
departures as those in Photos 35, 37, and 38, a behaviour quite inconsistent 
with regarding them as protoxylem. 
An essential similarity is thus seen in the small and large steles, though 
the actual construction differs in rhizomes of different size. It is usual to 
find the primary structure of a young plant or a branch continuing on the 
up-grade of elaboration until the adult type is reached and maintained. 
This is natural and intelligible whether the ontogenetic progression is to be 
looked upon as a phylogenetic recapitulation or as an expression of the 
cumulative increase in strength and nutritive power of the plant. The fact 
that in Helminthostachys the earlier grades in the progression may be 
maintained for many nodes of a rhizome, or, on the other hand, may be 
rapidly passed through, suggests that the physiological explanation may 
be the more important. This is further shown by the juvenile structure 
being resumed when rhizomes of adult type become more slender under 
unfavourable conditions of nutrition. Thus the usual progression must be 
regarded as an expression ol physiological differences, and its use as an 
index of phylogenetic history must be critically considered in this light. 
In Helminthostachys at least the physiological interpretation seems inevitable, 
whatever phylogenetic value may be attached to the different manifestations 
of the underlying type of construction. 
The essential structural fact appears to be the existence of two com¬ 
ponents in the xylem, inner xylem and outer xylem. Various indications 
suggest that this may be a distinction of fundamental importance in the 
general morphology of the conducting system of a shoot. Not to go into 
detail here, it may be mentioned that a similar distinction has been made in 
the case of the independently evolved conducting system of Mosses, 1 and 
that it is exhibited by the stele of the Lycopodiales, Equisetales, and 
Sphenophyllales. It is present in the stele of the adult stem in some Ferns 
(e. g. Gleicheniaceae) and is indicated in others. The distinction is clearly 
suggested in the slender basal region of the stems of many young Ferns. 
But the most interesting cases in the Pteropsida are found in a number of 
relatively archaic groups, such as the Osmundaceae, the Coenopterideae, 
especially the Zygopterideae, and also in some Cycadofilices. 
Leaving phylogenetic considerations out of the question for the present, 
the Ophioglossaceae appear to find their place with the last-named groups, 
as regards the general comparative morphology of the stele. The Ophio¬ 
glossaceae are of peculiar interest on account of the range of variants of the 
1 Cf. Tansley and Chick : Ann. of Bot., vol. xv, p. 1. 
