ON SOME WATER PLANTS.
111
is subjected to the flow of under currents, and the strain
that it has to withstand must often be great; indeed, fragments
of the leaves and of the brown creeping rhizomes are frequently
thrown up as jetsam, after storms, on the neighbouring
shores. Examination of the leaves shows that they are
strengthened without and within; the outer walls of the
epidermal cells are very thick, far thicker than in any other
ribbon-leaves we have been considering, and the delicate green
tissues within are traversed by many strands of tough elastic
fibres; similar strands of fibres are scattered also through the
cortex of the creeping stem.
The inflorescence of the Grass Wrack is an inconspicuous
spike of stamens and pistils arranged on one side of a flattened
axis, and enfolded by a membranous spathe, or sheathing leaf-
base. Fertilization takes place under water, within the protec-
tion of the spathe. The pollen consists of long slender rod-like
bodies, very different in appearance from the usual round pollen
grains. The dried curled leaves are much used as packing material
for glass and china. At Rotterdam I saw barges laden with the
crisp dark-brown masses, collected probably from the beds of
Zostera growing in the. Zuyder Zee. The distribution of Zostera
marina is wide, for it occurs on most temperate coasts.
Along the south coast of France there grows in some places
a plant closely allied to Zostera, which also has become adapted
to marine life; this is Posidonia Caulini. On the shore near
Hyeres winter storms tear up the leaves and stems, while the
waves pile them into high banks or roll the fragments into
curious fibrous balls several inches across.
The Floating Pond-weed, Potamogeton natans, is abundant
in the forest ponds, where the broad blades of the floating leaves
may cover the whole surface of the water. The narrow-leaved
variety, subspecies polygonifolius, is common in the Roding.
In winter, and always where the current is swift, no floating
leaves appear, but instead are to be found submerged leaves,
some with narrow translucent blades on long petioles, others with
long bladeless petioles, and others consisting of thin ribbon-
leaves.
And here I should like to refer briefly to a theory which
throws fresh light on the leaves of these Monocotyledonous
plants.