148 
DR JOHN M'LEAN THOMPSON ON 
structure of a leaf-trace (fig. 53), but as tbe point of branching is approached the 
stele widens and divides into two dissimilar portions (figs. 54, 55, and 56). The one 
is a small protostele, the other a typical leaf-trace. The protostele enters the small 
incomplete horizontal branch, which might or might not prove to be axis, but the 
leaf-trace passes on into the free rachial portion, and its typical structure is main- 
tained until the upper fractured surface is reached (fig. 57). 
The foregoing survey of the axis and rachis of this second specimen brings the 
latter well into line with the conditions shown by the first specimen. But reduction 
and abortion, which have modified the first plant’s structure, have more 
fundamentally affected the second specimen. Malnutrition may have 
been a potent factor, which had been superposed upon xerophytic life 
conditions ; and the form and structure of this second specimen bear 
eloquent evidence to the dictatorial nature of the environment. The 
anatomical study of the materials available has disclosed stelar and leaf- 
trace characters which lend strong support to a relationship between 
Stromatopteris and Gleichenia. It has been shown that drastic reduc- 
tions in the branch-sytem occur, and that these affect in particular the 
leaf-bearing branches and the leaves themselves. Stromatopteris may 
indeed be considered a very xerophilous and reduced type. 
It seems probable that the leaves of Stromatopteris are typically 
simply pinnate and unbranched, and the specimen available for study 
is of this unbranched type (fig. 58). The leaves of the plants in Kew 
Herbarium are all unbranched, and the Edinburgh specimen shows a 
similar condition. But branching of the leaf may occur, and an example 
of this is shown in the right-hand plant in fig. 59. No surprise need be 
felt at an occasional branching in this xerophilous type, if, as comparison 
indicates, it arose from a stock where the leaves are typically branched. 
Indeed, branched pinnate leaves occur in Platyzoma , in which reduction 
Text-fig. 16. ^ j ea £ ^ ag j 3 ec01Iie a dominant process, and, as I have shown 
elsewhere (Trans. Roy. Soc. Edin., vol. li, 1916), has led to the production of 
characteristic filiform leaves. 
Both fertile and sterile leaves are present in the Kew and Edinburgh specimens, 
and no difference in form is recognisable between them. The pinnae are universally 
smalL and leathery, with slightly revolute margins, and with stomata restricted to 
the lower surface. As will be seen from text-figs. 17 and 18, there is an alternation 
of the pinnae of the rows on either side of the rachis. The traces draw off from the 
rachis towards the basiscopic margins of the pinnae, and are universally of extra- 
marginal origin (figs. 60 and 61 ). The venation is liable to vary as to details from 
pinna to pinna, but is universally a sympodial dichotomy, in which the basiscopic 
member of the first dichotomy is usually dominant, and by its further branching 
supplies the greater part of the pinna. The acroscopic member remains unbranched 
