58 Calvert and Boodle.- — On Laticiferous Tissue 
bundle. The series of secretory sacs, described by Pax as 
articulated laticiferous tubes, have also been found to occur in 
the pith in certain of the Acalyphineae 1 . 
The pith of the stem and the parenchyma of the upper 
portion of the petiole are connected by thin-walled unlignified 
cells elongated in the direction of the leaf-bundle ; it seems 
probable that this tissue is continuous with the groups of 
‘ cambiform 5 cells occurring immediately outside the pith. 
The laticiferous tubes passing out to the leaf are usually 
found in this tissue 2 * . 
Behind the leaf-scar, bordering on the pith and protruding 
considerably into it, is a mass of lignified tissue, whose 
elements resemble, in form and arrangement, those of the 
ordinary pith (Figs. 4 and 6) ; their walls are however much 
thickened and pitted. The internal mass of the pith in the 
internodes and in the older nodes consists of larger cells, 
which have much thinner walls but are also slightly lignified, 
while the peripheral portion, as already described, has thin 
walls of unchanged cellulose. 
Transverse sections through young nodes show that the 
cambium-ring is incomplete behind the leaf-scar (Fig. 5) for 
some distance above the points where the leaf-bundles join 
the vascular cylinder of the stem, and that ordinary thin- 
walled pith-cells occupy, relatively, the same position as do the 
thick-walled lignified cells above mentioned, in the older stem. 
These lignified cells cannot, therefore, be a product of the 
activity of the cambium ; they seem to be ordinary pith-cells 
which have undergone secondary sclerosis. As there is less 
secondary wood formed behind the leaf-scar than in other 
parts of the stem, this lignified parenchyma may be needed 
for mechanical support. The laticiferous vessels pass through 
it, as through the thin-walled peripheral pith (Figs. 4 and 6). 
The mass of the pith in fairly young nodes resembles the 
outer portion of the pith of the internodes in having thin 
1 Engler’s Bot. Jahrb. Bd. v. p. 404. 
2 This tissue is not clearly shown in Fig. 4, which was taken from an old 
stem. 
