194 The Living Plant 



dissolved from the bodies of insects by the pitcher-plants or other 

 insectivora, is also of this character. In all of these cases the 

 materials are not drawn in, as by osmosis, but are driven in by 

 the energy of their own diffusion. 



In reviewing absorption by plants, the reader must be struck 

 by the fact that the forces at work are chiefly molecular, and 

 therefore slow and gradual, even though powerful in their action. 

 Plants, as it were, arrange the conditions to permit the molec- 

 ular forces to work for them. In this respect they stand in 

 rather marked contrast to animals, which tend rather to make 

 use of those larger or molar forces which permit greater rapidity 

 and range of action. In this difference we have the explanation 

 of the persistent placidity of plants in comparison with the 

 abounding activity of animals. 



This chapter is already so long that it is only with reluctance 

 that I add anything more; but there remain a few matters which 

 must receive some discussion in this immediate connection. First, 

 we must examine a little farther the arrangements for aeration in 

 plants, especially under unusual conditionSo Wherever particular 

 need exists, there the inter-cellular system may become much 

 larger, as occurs conspicuously in leaves, which, requiring a 

 carbon dioxide supply for photosynthesis ten tmies or more 

 greater than the oxygen supply they need for respiration, exhibit 

 a far larger aeration system than any part of the plant needing 

 only a respiration supply; and that is wh}^ leaves have the mark- 

 edly spongy texture they so commonly exliibit. Again, there are 

 plants of such habit that their roots (as in Marsh Plants), or 

 even huge rootstocks (as in Water Lihes), lie deep under water 

 and must be aerated in some way from the surface. In such cases 

 the inter-cellular system is immensel}^ developed, even to the 

 formation of elaborate passages, in the parts which lead from the 

 surface to the parts under water; and this is the reason for the 

 soft, open, spongy texture of the petioles of Water Plants, and 

 the pith of Rushes and Sedges, and it explains why some plants 



