Anatomy of Solenostelic Ferns : /. L ox soma. 87 
continuous with the upper surface of the solenostele, they 
disappear altogether without at any time coming into relation 
with the scalariform protoxylem-elements that line the ex- 
ternal border of the lower part of the solenostele. 
In the leaf-trace, therefore, the xylem is developed centri- 
fugally from definite and typical protoxylem-groups. On 
the other hand, it has been shown that in the stem the develop- 
ment of the xylem is not related to the leaf-trace protoxylems 
decurrent in the stem, nor to any other definitely localized 
groups ; neither are there any elements present in the stem 
which exhibit the characteristics typical for protoxylem. 
Since the whole mass of xylem in the stem cannot well be 
regarded as metaxylem (primary xylem other than proto- 
xylem), I find it necessary to accept the alternative, and to 
recognize in Loxsoma two kinds of protoxylem differing in 
structure and in disposition ; one peculiar to the stem, and 
the other to the leaf. Particular attention has been drawn 
to this distinction because it may prove a factor of considerable 
importance in the estimation of the influence exerted by the 
anatomy of the leaf upon that of the stem, in cases where the 
former is the predominant member of the plant, a subject 
which does not seem to have attracted the attention it deserves. 
The xylem is almost entirely free from parenchyma scat- 
tered among its tracheides, but at the same time the whole 
strand is surrounded by a well-defined layer of cells exactly 
similar to those of xylem-parenchyma. However, the cells 
of this xylem-sheath (Fig. 6 , x. sli) are not all alike, for those 
in the neighbourhood of the protoxylem-groups undergo such 
profound and peculiar modifications that they have been con- 
sidered to be a new kind of tissue. The cells at these points 
become greatly enlarged by growing out into the space left 
vacant by the disintegration of the protoxylem-elements ; 
their walls, which are considerably thickened and sometimes 
even lignified, are thrown into a number of deep irregular 
folds or pleats, especially on the side facing the protoxylem 
(Fig. 11, cp). They are covered by a number of large simple 
pits, often to such an extent that they appear to be coarsely 
