ARRANGEMENTS FOR RETAINING THE POSITION ASSUMED. 425 



of Water Milfoil, and of various others which lead a peaceful life in the depths 

 of calm waters. Their tissues are not feeble, but elastic and pliant, and many 

 sea-wracks look just like leathern straps and bands. Many of these sea-wracks 

 are periodically left lying on the dry ground at low tide, but they do not in con- 

 sequence shrivel, or at least not if the water soon returns, but lie with their 

 pliant leaf-like surfaces fiat on the dry sand or stone. When the tide returns, 

 they are again gradually raised up, and assume an upright position in the sur- 

 rounding water; and this is materially assisted in the sea- wracks by the swollen 

 bladder-like cavities, in reality swim-bladders, which they contain in their tissues. 

 Many species of Characese, but still more the Lithothamnese and Corallineae, 

 acquire an increased capacity of resistance against the force of the waves by 

 the deposition of lime in the cell-membranes; others again so closely apply their 

 large surfaces to the rocky reefs and stones of the shore, that they look like 

 coloured patches on them, so that the crushing or tossing effect of the surging 

 waves is entirely obviated. This applies, for example, to Hildebrandtia rosea 

 and Hildebrandtia Nardi, which cover the stones with blood-red patches. 



Many marsh-plants, which are only partially, and often only temporarily, 

 submerged, whose floating leaves are half in contact with water and half with 

 air, or whose leaf -blades are wholly raised above the water, behave just like 

 these water-plants. The alteration of the water-level brings about, of course, 

 a higher or lower position, an elevation and sinking of the floating leaves, but 

 this is effected without the slightest rupture of the parts in question. The stem 

 and the leaf -stalks, which proceed from a stock rooted at the bottom of the water, 

 resemble long strings and threads to whose upper ends the leaf -blades are fastened. 

 At the highest water-level the floating leaf-discs stand perpendicularly above the 

 stock to which they belong, which is rooted in the depths. If the water then 

 sinks, the leaves, floating on its surface, fall with it, and at the same time separate 

 from one another. The stalks and stem proceeding from a stock perform approxi- 

 mately the same movement as that seen in the ribs of an umbrella held downwards 

 and then opened. As soon as the level of the water rises again, the reverse 

 movement naturally occurs. Many of these marsh-plants, as, for example, the 

 Water Chestnut (Trapa), also possess air-bladders in the floating portions of their 

 leaves, having the same function as those of the sea-wracks. Moreover, usually 

 two kinds of green foliage-leaves are noticed in these. Submerged leaves, which 

 are constructed like those of aquatic plants, and floating leaves which display 

 a more or less disc-like form, and whose under side is in contact with the water, 

 the upper with the air, but which under certain circumstances may be entirely 

 surrounded with air without injury. If the marsh should dry up, long thin 

 stems and leaf -stalks would be anything but beneficial; the metre-long leaf-stalks 

 of a water-lily would not be able to support the leaves in an erect position, but 

 would fall and become bent. Stretched out on the ground, also, such long fila- 

 mentous leaf-stalks would not be advantageous. It is seen, too, that marsh- 

 plants of this kind immediately become modified when the water recedes. The 



