Tissues in Certain Monocotyledons. 27 
on passing the cambium, to the earliest condition of the des- 
mogen-strands. We can trace the first longitudinal divisions 
of the mother-cells from which these strands are formed. As 
is well known, it is usually a single longitudinal row of mother- 
cells, derived from the cambium, from which the entire strand 
is developed 1 , though sometimes two adjacent rows may 
contribute to its formation. As we advance inwards the 
young desmogen continues to show a beautifully clear and 
simple structure. The outlines of every daughter-cell are 
clear and sharp, and the limits of their mother-cells can still 
be traced. All the desmogen-cells at this stage are alike ; 
they are prismatic in form, with wedge-shaped ends fitting 
closely together. Each cell has a single nucleus, and all the 
nuclei are similar to one another. As we now proceed to 
examine sections rather further inwards, we find that the 
somewhat more advanced strands show a differentiation in 
two respects. (1) We notice every here and there an ex- 
ceptionally large nucleus, often much elongated, and sharply 
distinguished from the small nuclei of other desmogen-cells. 
(2) We find that the regularity of structure of the desmogen 
is now at some places interrupted. Most of the regular 
prismatic cells remain unaltered, but occasionally we find 
among them a cell with more pointed ends projecting beyond 
those of neighbouring cells. Sometimes the ends of these 
elongated cells cannot be traced at all as they dip out of the 
plane of section. These exceptional elements are invariably 
those with the specially large nuclei. At first they are very 
rare, but as we advance to older bundles they are found more 
and more frequently. But now they are hardly ever found 
complete in a single section. Their length, and somewhat 
curved course, renders this impossible, and it is only by the 
comparison of successive sections that we are able to build up 
the entire element. As we proceed to yet older bundles we 
find that the greater part of them is now formed of these long, 
irregularly curved cells. Ultimately we recognise the latter 
in a changed condition ; their walls are thicker, bordered pits 
1 Cf. Roseler, 1 . c. p. 322. 
