ON SOME WATER PLANTS. 
I I I 
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 r barges laden v r ith the 
crisp dark-brown masses, collected probably from the beds of 
Zostem growing in the Zuyder Zee. The distiibution of Zoster a 
marina is wide, for it occurs on most temperate coasts. 
Along the south coast of France there grow r s 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 polygonifolias , is common in the Roding. 
In winter, and always w r here 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. 
