with special reference to the occurrence of Amitosis, etc. 371 
irregularly placed. The inner region is differentiated into a 
lacunar part which may be called the middle cortex, and a com- 
pact part to which the name of inner cortex may be confined. 
The lacunar zone is characterised by large air spaces separated by 
radial plates of cells, as a rule only one cell wide in the tangential 
direction. These radial plates are continuous with the radial files 
of cells which make up the inner cortex. The origin of the lacune 
is of some interest. The whole inner region of the cortex must be 
visualised as consisting of radially arranged plates, one cell wide, 
which in the early stages are so placed as to leave no spaces 
between. The cells composing the plates divide very rapidly, and 
a number of new cell-walls are formed, all in planes at right angles 
to the long axis of the root. The result is that each plate elon- 
gates in the direction of growth of the root, but, owing to the 
rapidity of its cell-divisions, the plates grow in length faster than 
| 
: 
| the rest of the root, and are thus forced into undulations, since 
| 
they become too long to retain their normal vertical position. The 
possibility of their taking up this sinuous form is due to the fact 
that the root enlarges in diameter and thus allows room for the 
separation of the plates. It will readily be seen that a series of 
plates, side by side, elongating independently, and at the same 
time prevented from stretching to their full length, will naturally 
become detached from one another at certain points, leaving spaces 
between. The result of these processes is that the middle cortex, 
aS seen in transverse section, consists of radial plates of cells, like 
_the spokes of a wheel, separated by lacunz, whereas in tangential 
section the plates are found to meet their neighbours at intervals 
so as to form a network (PI. VIII, Fig. 2). 
__ The structure of the mature root of Stratiotes aloides has been 
described by Van Tieghem and Douliot*. These authors must 
have selected an unusually small specimen for study, for they 
describe the root as pentarch, whereas I have found as many as 
eight protoxylem elements and about eight metaxylem vessels 
alternating with eight phloem groups, each consisting of one to 
three sieve-tubes with their accompanying companion cells. Lignifi- 
| cation is extremely slight; the metaxylem elements bear delicate 
‘Scalariform thickenings. 
| 
In young roots the embryonic vessels are represented by files 
of large elements with correspondingly large nuclei (PI. IX, Fig. 6). 
The young sieve-tubes are narrower in lumen than the young 
vessels. They consist, at an early stage, of segmented tubes, 
poor in contents and without nuclei. The segments are shorter 
_than in the case of the young vessels, and the partition walls are 
horizontal instead of oblique. The accompanying cells are not 
typical companion cells, since they are much shorter than the 
* Le. p. 337. 
25—2 
