Dar bishire. — Observations on Mamillaria elongata. 381 
living cells, and they are practically of the same breadth and length as the 
tracheids. In breadth they are not unfrequently compressed, but their 
length may be said to be identical with that of the lignified cells. 
Another kind of cell is found in the small plants of M. elongata 
towards the lower end of the main stem. In the later additions to the 
metaxylem we get in the latter, apart from the large spiral tracheids and 
parenchymatous cells, some libriform cells, which have very much thickened 
walls. In length they are about equal to the spiral tracheids, but in the 
region where they occur, the position of the tracheids with regard to one 
another has become rather irregular and they may be longer than at other 
points. The libriform cells measure up to about 90 and 120/x in length 
(PI. XXVI, Fig. 25, q). The transverse walls of the neighbouring cells still 
remain at about the same level. The libriform cells contain cytoplasm 
and nucleus, both of which can be made out clearly in transverse and 
longitudinal sections (PI. XXVI, Fig. 26). The cells are angular in outline, 
and measure about 15 to 30 [i across (PI. XXVI, Fig. 24). The thickening, 
which is continuous except for a number of pits, is not more than 4-5 /x 
in depth. The walls of these cells are flat and not rounded even when 
abutting on a parenchymatous cell. They have very distinct crossed pits. 
The opening of the pit towards the cell-cavity is a long and narrow slit, 
which widens out towards the middle lamella. At the latter point, the pit 
may be 2 m across. The slit-like opening to the cell-cavity is about 3-4 ju, 
long. The slit in any given cell lies at right angles to the corresponding 
slit of a neighbouring cell. The pits are found on all the walls (PI. XXVI, 
Fig. 26). 
The cells of the ground-tissue, which immediately surround the bundle, 
are very much compressed (PI. XXVI, Fig. 21), so that their cavity is often 
very small as compared with their whole bulk. 
(b) Anatomy of the Root. 
The few small plants of M. elongata which I have examined were 
evidently grown from seeds. The whole root-system in these consisted of 
a distinct tap-root, with a small number of lateral roots. The former 
extends into the soil for a distance which is about equal to the height of 
the shoot above the soil. 
The tap-root is usually very much thicker than the lateral roots, more 
particularly near its junction with the shoot (PI. XXV, Fig. 3). 
The structure of the root-tissues is very simple, but shows several 
important differences as compared with the shoot, apart from its general 
arrangement as a normal dicotyledonous root. 
The growth of the root-tip and the origin at this point of the various 
tissue systems has been described by v. Breda de Haan for Melocactus (4, 
pp. 9-1 1). Pretty much the same condition of things, I imagine, will obtain 
