242 POPULAR SCIENCE MONTHLY. 



an aqueous solution outside the cell, can there be any absorption. The 

 submersed aquatic has many or all of its cells in direct contact with 

 the water. The land plant has only those cells which touch, or are in, 

 the soil which are regularly in direct contact with water. Except those 

 plants living in swamp or marsh, and except immediately after heavy 

 rain, land plants are able to obtain only those thin films of water 

 held on the surfaces of the soil particles. To reach these films, to 

 bring the solution within the cells into contact with the water (also a 

 solution) on the soil particles, land plants develop hairs — the rhizoids 

 of the lower forms, the root-hairs of the higher. An aquatic com- 

 posed of a chain or of a film of cells has all its cells directly in con- 

 tact with the water, which holds in solution oxygen, carbon dioxide, 

 and those mineral salts which constitute its food materials. An 

 aquatic composed of a mass of cells, on the other hand, has only some 

 cells which are able directly to absorb food materials from the water, 

 those cells on the surface. The surface cells constitute the absorbing 

 organ. Under these are other cells, containing chlorophyll, which 

 manufacture the absorbed food materials into foods. If the plant is 

 small, there may be besides only those cells which are used for storing 

 the manufactured product and those concerned with reproduction. If 

 the plant is larger, like the rock weeds and kelps, there must be in 

 addition a system of cells for conducting the foods from the cells 

 manufacturing them to others needing them. In all aquatics, even the 

 largest, unless some are land plants retaining the structures charac- 

 teristic of land plants even after becoming aquatic, there is only this 

 one system of conducting tissues, the one which distributes food. 



As we pass from the submersed aquatics to those only periodically 

 submersed, from these to plants living prostrate on the ground, like 

 most liverworts, and from these to erect plants, we see progressive 

 changes in absorbing and conducting systems. The plants living 

 between the tide-marks, for example the rock weeds and devil's apron 

 (Lamviaria), possess a conducting system similar to the submersed 

 kelps, but the absorbing system is reduced in extent to prevent the 

 plant from losing water by evaporation while exposed at low tide. 

 Jn these plants there is need of two sets of qualities, those adapted to 

 life under water, those fitted to life in the air — essentially, enough 

 cells for absorbing water, and enough cells so placed and of such 

 composition as to keep evaporation within safe limits. 



The prostrate land plants, for example the liverworts, possess tis- 

 sues similar to the small though massive algge living between the 

 tide-m&,rks — an absorbing system and a protective system. But as, 

 for most of the time, the prostrate land plant can absorb water 

 only from the soil underneath it, and lose water by evaporation 

 only from its upper surface, the absorbing and protective systems are 



