312 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
Once formed, the growing point extends itself along the 
rock pretty rapidly. I have never been able to find that the 
hypocotyl gives rise to more than one growing point, but in 
the absence of large quantities of material it would be rash 
to dogmatize. It will be well to point out at this stage one 
of the many traps into which one is liable to fall in working 
at these plants. Frequently I have obtained what I thought 
to be seedlings, but which on thorough investigation on the 
spot proved to be rejuvenescences of old exposed portions 
of thallus, which, as we shall see, are very common. The 
new growing points formed from these old thalli are exactly 
like seedlings, and still more so when, as soon happens, the 
old thallus from which they sprang shrivels away and is 
washed off by the current. 
The next stage that I have found is one about half way to 
that figured in PI. IX., fig. 12, in which owing to branching the 
seedling now has seven growing points. It will be observed 
that the dimorphism of the leaves, which was beginning to 
show itself even in the minute seedling of fig. 9, is now very 
distinct ; there are large ovate leaves on the edges and 
smaller linear ones on the upper surface of the growing tip, 
the former in a single row on each side, the latter in a dense 
irregular crowd. This is well shown in PI. IX., fig. 10, which 
represents a particularly distinct case sometimes found. 
The marginal leaves are parallel to each other, each with its 
lower and anterior edge overlapped by the upper and pos- 
terior edge of the next younger leaf. At the very tip the 
lateral leaves are more or less upcurved, to some extent 
covering the apex. 
PI. XI., fig. 5, which is slightly diagrammatic, shows a 
longitudinal section through a growing point in October. 
The ‘‘apex,” or perhaps rather growing margin, of the 
thallus is asymmetrical, the youngest cells being a little short 
of, and above, the extreme tip, as shown at m. On the 
under side of this tip there is a layer of collenchymatous 
cells (coZ.), which (in the flabelliform thalli, at any rate) 
split off as the growing point expands, and thus appear to 
