176 BOTANY. 



229. The Compounds Used. With the single exception 

 of oxygen, the elementary constituents named above do 

 not enter into the food of plants in an uncombined state ; 

 on the contrary, they are always absorbed in the condition 

 of compounds, as water, carbon dioxide, and the 



In addition to these, many organic compounds are ab- 

 sorbed in particular cases, as in those plants which live in 

 decaying animal or vegetable matter (saprophytes), as well 

 as those which absorb the juices from living plants (para- 

 sites). 



230. How the Pood is Obtained. In the case of aquatic 

 plants, these compounds are taken into the plant-body by a 

 process of diffusion from the surrounding water ; in terres- 

 trial plants the gaseous compounds, as carbon dioxide and 

 carbonate of ammonia, are absorbed at least in part by the 

 leaves directly from the surrounding air, while the solutions 

 of these and the other compounds in the water in the soil 

 find their way into the plant by diffusion. 



230. How the Pood is Transported in the Plant. 

 Once within the plant-body, the food materials diffuse to all 

 watery parts, in the case of the larger terrestrial plants ris- 

 ing through the stem to the leaves. By diffusion, there is a 

 constant tendency toward an equal distribution throughout 

 the plant of the solutions which enter it, and if there were 

 no disturbing chemical reactions taking place, such a condi- 

 tion would in most plants be soon reached. It is quite 

 probable, indeed, that this actually happens for certain sub- 

 stances which are found in solution in the soil or water, and 

 which, entering plants, diffuse through them to all parts, 

 but not being used they soon reach a state of equal diffusion, 

 which is only slightly disturbed by the extension of the 

 plant-body by growth. Doubtless the rapid diffusion of 

 food materials throughout terrestrial plants is aided by the 



