Hymenophyllaceae, Schizaeaceae and Gleicheniaceae. 533 
regard the pith as not belonging to the stele. They recognize 
the difficulty in estimating the morphological nature of a 
tissue, and state that ; our criteria only become applicable 
as the adult condition is reached 7 or approached. 
A thoroughly consistent and strictly morphological treat- 
ment of tissues is probably an impossibility, and in any case 
the subject is rather elusive, but in many cases one can draw 
an opinion from the position of a certain tissue, though 
suppositions as to the exact mode of its first origin may 
become necessary. Two cases may be brought forward in 
which the morphological nature appears fairly certain. The 
cortical sieve-tubes of Cucurbita must be regarded as derived 
from cortical cells ; morphologically they are part of the 
cortex. Secondly, the trabeculae in the sporangium of 
Isoetes have probably been derived from the sporogenous 
tissue and, morphologically speaking, represent part of it, 
— not ingrowths of the surrounding tissue. 
We will now turn to the stele. It is exceedingly probable 
that the solid protostele was the universal primitive type, that 
the more complicated types were moulded from it, and 
that it never passed through a stage of flattening and rolling 
round, such as is assumed by the writer for the petiolar 
bundle of many Ferns. Consequently, whatever tissue is 
found within the xylem is presumably morphologically stelar. 
Assuming an exarch protostele, a pith may have originated 
by incomplete differentiation of the xylem-mass. At any 
rate if one regards the pith or other central tissue as having 
arisen in the first place by the transformation of potential 
tracheides into other tissue elements, these latter should 
be treated morphologically as part of the stele. 
The different types of stem-structure in Ferns have probably 
been derived by a differentiation of the protostele into vascular 
and non-vascular parts, hence, although the possibility of 
there being exceptions is kept in view, the writer agrees with 
Schoute’s conclusion (’ 02 , p. 163) as a provisional generaliza- 
tion that a single type of stele is found in the stem and root 
of the vascular plants, viz. monostely ; that is to say, taking 
O o 2 
