344 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
To turn for a moment to the internal structure of the 
thallus at this stage, we find several features of interest, 
details of which must as usual be postponed for a later paper. 
In transverse section the thallus PI. XVIII., fig. 6) is 
approximately circular or elliptical, with a single vascular 
strand towards the lower side, surrounded by a cortex whose 
cells are collenchymatous towards the centre, and with walls 
thinner towards the epidermis. The vascular strand is 
elongated in the horizontal plane (fig. 8), and near the tip of 
the thallus is only two or three cells thick. At the ends may 
be seen two small-celled groups, which may perhaps re- 
present primary xylem, and the rest of the tissue is paren- 
chymatous or phloem-like, especially on the upper side, where 
the cells are narrower, and seem to represent primary 
phloem ; no sign of any characteristic xylem markings 
or of lignification is present. Further down in the thallus 
meristematic activity is evident in the vascular strand. The 
rather larger lower cells of the strand are found to be 
dividing, chiefly at first in the horizontal plane, but later 
also in the vertical or irregularly. A commencement of this 
process is shown in fig. 9, and two cells, with their resulting 
products, are shown in fig, 10 at a stage when the process is 
nearly complete, and when the bundle has reached the stage 
figured by Warming from January material. Besides this 
growth in thickness in the bundle, there is also a corres- 
ponding growth in the cortex. As shown in fig. 7, it begins 
in the outer layers and on the upper side, and is at first 
tangential, but very soon most of the cortex begins to 
divide both tangentially and radially. Even at this stage, 
though the bundle has reached some size, there seems to 
be no lignification, nor any characteristic xylem elements, 
though these may be seen later in the lower parts of the 
bundle. 
The heavy rains of the north-east monsoon now begin, 
and by December, when the plant is again in shallow water, 
the development of the flowers is practically complete, 
though there is no sign of it in October. As might be 
