92 DIFFUSION AND OSMOTIC PRESSURE 



cete plasmodium lies. Thus, if the plant body is not exten- 

 sive and is mostly in contact either with free aqueous solu- 

 tion or with some moist substance, the problem of how it 

 obtains water is a simple one. Also, mere diffusion from one 

 part of the organism to another, and that for only short dis- 

 tances, is sufficient to account for all the water transport to 

 be met with in such plants. 



But where the plant body elongates upward from its sub- 

 stratum (a phenomenon occurring in all forms, from the 

 sporophores of fungi to the stems of higher plants), it 

 becomes more difficult to point out exactly how every living 

 cell is kept saturated with water. In the case of fungi the 

 rhizoidal part of any filament is in direct contact with the 

 substratum, and here the solution of the substratum is con- 

 tinuous with that of the protoplasm, through the saturated 

 cellulose membrane. The sporophore is in connection with 

 absorbing rhizoids and through these it absorbs not only 

 what water is needed for growth, metabolism, etc., but also 

 what is needed to replace that lost by evaporation. For if 

 the loss by evaporation is not made up, death, or at least 

 suspension of activity, must ultimately ensue from desicca- 

 tion. The cellulose walls which are exposed to the air are 

 more or less thickened, thus causing water loss by evapora- 

 tion to be much less pronounced than it would otherwise be. 

 Indeed, fungi are seldom found growing in localities where 

 evaporation is very marked. In the case of algal filaments, 

 which grow upward from the substratum, and in that of ele- 

 vated capsules in liverworts and mosses, water transport is 

 to be explained in the same way. 



In the higher green plants, except in the case of aquatic 

 plants, conditions become much more complex. Here the 

 most active parts, the leaves and expanding buds, are often 

 removed many meters from the soil out of which the supply 

 of water must come. In these plants special organs of 



