EXTERNAL CONDITIONS ON PLANT LIFE. 179 



film of water, which is forcibly taken from it by the osmotic 

 action of the root hairs. Each hair is an elongation of a 

 single epidermal cell. Its external wall is composed of porous 

 cellulose. Besides the ordinary cell contents, such as nucleus, 

 masses of living protoplasm, vacuoles, microsomata, and the 

 like, there is a thin semi-fluid layer of protoplasm, closely 

 appressed to the whole interior of this porous cellulose wall. 

 This forms, with its cellulose support, an osmometer, and 

 when the root hair is tightly crowded in its growth against 

 the surrounding particles of earth, thus coming in contact 

 with the water film, there is at once set up, with great force, 

 a current from the thin fluid surrounding the particles 

 through the walls to the more or less thickened one within. 

 Under a pressure often of nearly an atmosphere, this water 

 is forced from the exterior cells of the root to the interior 

 conducting nbro-vascular bundles, through which it finds its 

 way to every part of the plant. It will be seen that the 

 condition of this forcible absorption is the interposition of 

 a protoplasmic membrane between two fluids of different 

 densities, the more dense, toward which the current always 

 tends, being within the cell. It is only necessary to state 

 that in sea-water we have a fluid of even greater density 

 than is usually found within the cells of plants, and that 

 this would tend at once to make it either very difficult or 

 utterly impossible for the plant to secure its water. Plants 

 which have adapted themselves to the influence of sea- water 

 have done it in a number of ways sometimes by taking large 

 quantities of salt into the cells, which balances much of the 

 salt without. In this way the density of the fluid within is 

 made greater than that of the water without, and absorption 

 takes place to a limited extent. But since it is still very 

 difficult for the plant to secure a large amount of water, 

 although it may stand in it, " Water, water everywhere, and 

 not a drop to drink," like Coleridge's shipwrecked mariner, 

 all the other methods of retaining its scanty supply, usually 

 found in desert plants, are here also exemplified. Thus, the 

 leaves may become smaller or thickened or disappear alto- 

 gether, in order to lessen the transpiring surface. If the 



