384 WILLIS : MORPHOLOGY OP THE PODOSTEMACEÆ 
rule, that I was able to find them under the water. All 
show a thallus more or less developed, and one or more 
haptera. Where the thallus has developed exogenously from 
the hypocotyl, it is only to be distinguished from the hap- 
tera, so long as it remains small, by the structure of its apex 
or rather margin, which is on the whole broader, and more 
evidently meristematic, the cells being smaller and more 
numerous. The thallus nearly always starts in a downward 
direction. It seems most frequently to arise at right angles 
to the plane of the cotyledons, as the figures show. 
By means of the haptera and the thallus, both of which 
follow all irregularities of the substratum and develop 
rhizoids on the lower side, the seedling is very firmly fasten- 
ed to its support, and cannot be washed away by any force 
of the water. The haptera seem usually to remain small, 
whereas the thallus soon reaches a considerable size, and by 
the end of the year may be a foot in diameter. The primary 
axis is usually bent down by the force of the water, and the 
cotyledons also often twist round to such an extent that the 
hairy upper surfaces face downwards. 
The first two leaves soon appear between the cotyledons, 
in a plane at right angles to that of the latter, or nearly so 
(fig. 13). They appear in alternate order, but very nearly 
at the same time (fig. 12). The next two are approximately 
at right angles to the first (fig. 13). The primary axis 
continues to grow in length and thickness while this is 
taking place. The leaves continue to appear at the apex for 
some considerable time, and the axis to increase in size to 
correspond (PI. XXXIY., fig. 3), till at last, towards the end of 
September, it may be as much as 5 cm. high and 3 mm. thick 
with a large number of leaves at its apex, which themselves 
may be 10 cm. long (Pi. XXXIY., fig. 4). When a number of 
leaves have been formed, the phyllotaxy seems to be of the 
I or type, but I have not been able to make this out in a 
satisfactory way from want of material, and I do not think 
the point of very great importance. PL XXXII., figs. 15, 16, 
shows a young plant with its leaf arrangement. In a short 
