Ti 'is tic tui A iter nifolia in Egypt . i 7 
Each branch of the thallus is traversed by a vascular strand, 
the xylem lies on the ventral side, and is represented by a few long 
annular or spirally marked tracheides; dorsal to these are the long 
narrow sieve tubes with their companion cells. In older thalli these 
become obliterated. Professor Warming describes a well developed 
root-stele of the Podostemaceae as being formed of two collateral 
bundles, with no alternation of xylem and phloem, which almost fuse 
together in the middle line to form a monarch system, the xylem 
occupying the ventral and the phloem the dorsal side. In this 
Egyptian specimen the double character of the vascular strand could 
not he detected. The long slender stems are either simple, or have 
one or more branches which originate as axillary buds: the vascular 
bundle is here a strand of long narrow cells that show no spiral or 
annular thickenings. 
In each group of leafy shoots one shoot is often shorter and 
more procumbent than the others, while its leaves are more closely 
imbricated and distinctly arranged in the three rows characteristic 
of the genus Tristiclia , one row being dorsal and of short leaves, the 
two lateral rows being of longer leaves. In the ascending stems the 
leaves are more irregularly scattered; the lower are rounded or 
oblong, hut they gradually become longer as they ascend, and those 
forming the terminal tuft are narrowly linear and measure 3*5 mm. 
in length by 0*2 mm. in breadth. Each leaf has a midrib, formed of 
several layers of cells, extending more than half-way; the remainder of 
the leaf consists of a single layer of rectangular or long hexagonal cells 
containing chlorophyll whose green colour is masked by red colouring 
matter. Here and there, on either surface of the leaf, short cells 
are met with cut off obliquely from the distal extremity of the 
longer cells; they contain granular protoplasm and show a conspicuous 
nucleus. Siliceous bodies are often deposited in these cells, and 
when treated with chromic acid appear as strongly refracting H-, V- 
or Y-shaped objects with deeply crenulated outlines: they are also 
present in small tooth-like cells along the margins of the leaves, but 
the Egyptian specimen is singular, compared with other examples of 
Tristiclia altemifolia in the scantiness of these siliceous deposits. 
The winter of 1900 no doubt afforded very unusual conditions 
of exposure to a delicate plant like the Tristiclia , for the Nile was 
lower than had been recorded for forty years. As a rule the 
river is lowest at the end of May; in June it begins to rise and 
attains its full height about the end of October, which at Assouan 
amounts to a rise of forty feet above the level in May. Thus at the 
