i6o 
WATER-PLANTS 
latter contains more oxygen than is needful, and less carbon 
dioxide than would be most advantageous to vegetation, the 
water-plant is highly favoured. Correlated with these things 
is the rapid growth, frequent branching, and extensive 
vegetative reproduction exhibited. This last occurs to some 
extent in the formation of several winter-buds or tubers on 
a single plant ; it happens also by the decaying away of the 
older parts of stems and the consequent liberation of the 
branches seen in so many water-plants, or even by the 
breaking off of twigs by currents of water or otherwise (as 
in Elodea), and in other ways. 
Correlated with the fact that all submerged parts absorb 
fluid directly is the absence of cuticle on the surface and 
the branching of leaves to expose a large area, while owing 
to this and to the fact that there is no transpiration in 
submerged plants, there are few or no stomata and little 
or no water-carrying xylem tissue present, and the root, 
deprived of one of its great functions, is most often either 
absent or much reduced, serving only for anchorage. 
Again, mechanical influences come into play. An 
ordinary stem or leaf requires a quantity of “ mechanical 
tissue ” (fibres) so disposed as to resist the stresses caused 
by weight, wind, and so on. In a water-plant the weight 
is upheld by the water and there is no need for mechanical 
tissue, nor do we find it. What little strain there is on the 
stem or leaf is usually longitudinal, and the vascular bundle 
(the strongest tissue of the plant) is axially placed, as in a 
land-root, to resist it. The leaf exhibits four main types, 
according to the conditions under which it lives, viz. 
(i) the ribbon type, (2) the much-divided type, and (3) the 
awl-shaped type in submerged leaves, and (4) the floating 
type. The first is seen in most Monocotyledons, e.g. 
Vallisneria, Glyceria, Potamogeton sp. &c. — the narrow 
leaf drifting with the current: in the form of a long ribbon. 
The much-divided leaf, with linear segments, offering little 
resistance to the movenaent of the water, is seen in 
Ranunculus, Cabomba, Trapa, &c. ; special patterns that 
also belong to this class are the leaves of Aponogeton 
fenestralis and others. Hippuris may form a link between 
this group and the preceding. The awl-shaped leaf is 
found in Isoetes, Subularia, Lobelia, &c. ; most of these 
