422 Scott and Hill.—Structure of Isoetes Hystrix . 
much pitted, the thicker bands of membrane, between the pits, 
forming a lattice-like reticulum (see Figs. 7 and 12). The 
pits are often subdivided by fine bars, into smaller areas. 
Little of the nature of formed contents can usually be de¬ 
tected, but sometimes small, deeply staining globules are 
found adhering to the walls, and apparently localized at 
the pits (Plate XXIV, Fig, 16). In the older parts of the 
stem the phloem is to a great extent obliterated, dense 
masses of callus-like substance appearing on the cell-walls, 
and almost fitting the cavity (Fig. 17). These masses stain 
like callus with coralline-soda, but the other callus-reactions 
tried did not give wholly satisfactory results, and, unlike true 
callus, these masses are apt to shrink away from the cell- 
walls. In their deep staining with haematoxylin they agree 
with the true callus in the sieve-tubes of the foliar bundle. 
We have not investigated the more minute histology of the 
phloem, and thus have not demonstrated the perforation of 
the thin-walled areas. That may be left to other investigators, 
but in the meantime, we can scarcely doubt that these enu¬ 
cleate elements, with the characteristic areolation of their 
walls, and their agreement in various reactions with the 
sieve-tubes of the leaf, with which, as we shall see, they 
are continuous, are best to be regarded as themselves repre¬ 
senting the sieve-tubes of the stem. In any case we must 
apply the name phloem to them exclusively, in describing 
the stem-anatomy. 
Russow detected these elements as long ago as 1872 (p. 139) 
and described them quite clearly. Fie says :— £ The tabular 
or shortly prismatic cells have clearly thickened and finely 
pitted walls, and in transverse section make quite the impres¬ 
sion of sieve-tubes or latticed cells in Coniferae: in their 
function they certainly agree with the bast-elements mentioned : 
their difference in form from sieve-tubes is explicable by the 
conditions of growth of the organs in which they occur/ 
Considering that Russow worked with herbarium-material, 
the accuracy of his description is wonderful, and it compares 
extremely favourably with later statements. Fie does not, 
