68 o Marsh . — The Anatomy of some Xerophilous Species of 
the median one of which, in the main part of the petiole, is located in the 
position shown in diagram 3 of the series in Fig. 8 . This is a condition 
which perhaps shows an unfulfilled tendency towards exarchy of the 
central protoxylem, an end which has become realized in the species of 
Cheilanthes. 
These facts admit of only one simple explanation, viz. that the base of 
the leaf preserves primitive characters. This is the position which has been 
taken up by Sinnott (7). He has pointed out that the leaf-trace of all Ferns 
except Osmundaceae, Ophioglossaceae, and Marattiaceae can be derived from 
C. Fend leri & = protoxijlam. C. persica 
Fig. 11. Series of transverse sections of the xylem of the petiolar traces of the five Ferns. 
Each series is numbered in acropetal order. * represents in each case the structure prevalent 
throughout the main part of the petiole. No. 8 in the Ch. Fendleri series shows the arrangement 
just below the first pinna. 
a triarch-mesarch trace, and that the nearest approach to this condition is 
usually found at the leaf-base. The mesarch protoxylems have in the 
course of evolution generally become endarch, and in very many forms 
all trace of mesarchy is lost, even at the leaf-base. This is the condition 
found at the base of the petiole of Ch. Fendleri , and it is only by assuming 
that the acropetal succession recapitulates the evolutionary succession that 
we can reasonably bring the peculiar petiole of Ch. Fendleri into line with 
the normal type of filicinean leaf-trace. Though it is difficult to imagine 
what could be the actual cause of the exarchy and detachment of the 
median protoxylem, it is simple, from a study of the leaf-base transition, to 
