Dar bishire. — Observations on M ami ll aria elongata . 383 
narrower ones are generally found nearest the centre of the root (PI. XXVI, 
Figs. 31, 32). 
The projecting ring of thickened wall-substance is a very conspicuous 
feature. It may project as much as 8 ju all round, but generally only as 
much as 5-7 \x. The separate rings are regularly about 3 \x high and 
separated from one another by a distance of about 18-20 /x. In cases 
where an annular tracheid adjoins a parenchymatous cell the tracheidal 
wall collapses slightly, thus becoming concave towards the cavity of the 
former (PI. XXVI, Fig. 32). 
A layer of cambial cells, slightly compressed radially, surrounds the 
metaxylem, but does not seem to be very actively dividing opposite the 
primary medullary rays (PI. XXVI, Fig. 31, r). 
The elements of the bast are of the same structure as already described 
for the main axis of the shoot. Small groups of protophloem (o) are 
dispersed in between the large cambiform cells (/). 
Then follow a few layers of large cells, which form the cortex. Their 
diameter is 18-30 /x in a radial direction, and 40-50 p in a tangential 
direction. Their length seems to be about 150 /u. They are of course 
living cells. 
The outer portion of the root is made up of numerous layers of cork- 
cells. Most of these have collapsed in a radial direction and can only be 
roughly counted (PI. XXVI, Figs. 31, 32). The cork is about 10-20 layers 
deep at the most. The size of the cork-cells is very uniform. They are 
70-80 fx long, and about 50-60 /x broad in the tangential direction of the 
whole root. Their radial diameter is often almost nil, owing to compression. 
It does not exceed 24 /x (PI. XXVI, Figs. 31, 32). 
In the young root of about 1000 /x in diameter just described the 
different tissues make up on the average the following proportions of any 
given radius : — Cork 40 ju, cortex too /x, bast 40 /x, wood 270 /x, the remaining 
tissues to the centre 60 /x. 
An older root higher up, just before it passes into the shoot, does not 
differ much from the thinner root just described. But certain structures are 
found, which we have not yet met with, at least in the root. 
At the point where the root passes into the stem there occur in the 
wood-part of the bundle small masses of libriform cells, already mentioned as 
being found near the older and lower end of the vascular bundle of the stem. 
The presence of these libriform cells seems to be characteristic of the lower 
end of the stem and upper portion of the root. The roots showing these 
libriform cells seemed to be i°5 to 2 mm. in thickness. Quite large 
quantities are found at these points in the stem and main root. In smaller 
adventitious roots I have never, however, seen them pass into the root-tissue 
except for a distance of 10-15 /x, though they may be found plentifully in 
the continuation of the latter into the stem. In the larger main roots I 
