784 Thomas . — Notes on Cephateuros. 
transparent and considerably larger than the small dark cells of the 
/ 3 form. 
The Alga on the leaves sent from Barbadoes is distinct from the 
Ceylon plants, both in structure and mode of growth. 
a . Large-celled Form (Ceylon). 
The Alga is always found on the upper surface of the leaf, from which 
it is easily detached as it is extra-cuticular. In the early stages the thallus 
is circular in outline, evidently arising from a rounded spore ; young stages 
have been seen in which the thallus is very small, not exceeding 001 111m. in 
diameter (Plate LIX, Fig. 1). 
Dividing walls are formed early ; they arise as ingrowths of the outer 
wall. The appearance is unlike that described by Marshall Ward, who 
states that when the zoospore comes to rest, the resulting cell begins to grow 
and divide, forming cell-walls, ‘ the pale lines at first seem to be isolated in 
the protoplasm, but their ends become at length united in the centre, 
and before long also at the circumference of the mass.’ 
In the material of this investigation a large number of early stages 
occurred, and the following sequence of events was observed. The Alga at 
first consists of a single circular but flattened cell ; as this enlarges cell- 
plates make their appearance at the periphery, and grow towards the centre 
of the expanding organism. Nine or ten, and even more, of these radially 
arranged plates may be observed, before two of them join at the centre to 
form a complete septum. The resulting disc, still retaining its circular 
outline, grows by the enlargement and further division of the peripheral 
cells, and transverse septa are formed. 
As the thallus grows still more, it tends to lose its circular outline. 
Contact with any obstacle arrests growth at that point, and the thallus 
spreads out, assuming an irregularly lobed appearance. If in their growth 
two thalli come into contact, the one becomes fused with the other, and both 
thalli continue their growth at all points, except where they are in contact 
with each other. 
Examination of a single lobe of the mature thallus suffices to show the 
typical structure. The cells of the Alga are arranged in a radiating fashion 
from a common centre, this is the result of the mode of growth. The 
growth is marginal; it may be conceived to have been derived from a con- 
dition such as that of Coleochaete , where the end cells of the filaments branch 
in a dichotomous fashion. In this organism, as the marginal cells grow out- 
wards, they divide by means of a cell-plate from the periphery towards the 
centre of the disc, thus tending to divide the cell radially into two segments. 
This wall never divides the cell completely, its development ceases when it 
has attained a length equal to about half the length of the cell. These 
