116 NEW ZEALAND PLANTS. 
light like diamonds. An individual plant is quite small, and consists 
of a thin, much-branched stem, which puts down roots into the water 
from its under-surface, and bears overlapping leaves. Hach leaf 
consists of two lobes, an upper and an under one, which, except on 
close examination, look like separate leaves. Hach lobe is adapted 
for a totally different condition of life, so there is a distinct division 
of labour in the one leaf. The upper lobes overlap one another like 
tiles on a roof; they are comparatively thick, provided with leaf- 
green, and therefore carbonaceous-food producers, and they are never 
submerged. Each contains a large cavity full of slime, inhabited by 
a fresh-water alga, named Anabaena, belonging to the Nostoc family, 
which, however, does its host no damage, but, like a respectable 
lodger, probably pays for its accommodation. The under lobe is 
partly submerged, quite thin, and able on its outer side to absorb 
the necessary water. Moreover, the close arrangement of the leaves 
as a whole furnishes cavities where air can lodge, and thus provides 
the necessary buoyancy for the floating plant. 
In most parts of New Zealand may be seen, floating on the 
surface of slow-ffowing rivers or calm sheets of water, the oval 
brown leaves, about 1 to 3 inches long, of some species or other of 
pond-weed (Potamogeton), the most abundant of which is the common 
pond-weed (P. Cheesemanii). Besides the above leaves there are 
others (but not in all the species) which live always submerged, and 
differ considerably from the floating ones. These submerged leaves 
are very thin, translucent, erect, more or less ribbon-shaped, extremely 
numerous, and always moving, no matter how gentle the current. 
Since there can be no danger of want of water at any time, such leaves 
are entirely without any protection on that score; on the contrary, 
they are so constructed as to be able to absorb water over their whole 
surface just like the filmy ferns already dealt with, and thereby secure 
at the same time the oxygen which the water contains. Their 
ribbon-like shape is well adapted to withstand damage from the 
currents of water, while sufficient extent of leaf-surface is provided 
by increase in number of leaves. It is also an interesting fact that 
these submerged leaves are similar to the early seedling ones of 
the pond-weed, and that this particular shape of leaf is common 
even amongst the land members of that great division of plants 
(monocotyledons) to which Potamogeton belongs. Some of the pond- 
weeds never produce floating leaves—e.g., Poltamogeton ochreatus 
