76 BOTANY. 



130. There are other elements which are made use of by 

 plants, but, as liie may be prolonged without them, they are 

 regarded as of secondary importance. In this list are Phos- 

 phorus, Calcium, Sodium, Magnesium, Chlorine, and Silicon. 



131. 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 



Of the last the nitrates of potash and ammonia, sulphate 

 of lime, carbonates of ammonia and lime, are probably to 

 be considered as the most important for ordinary plants. 

 "Water is necessary for all plants, and carbon dioxide for 

 those which are green. 



132. In addition to the foregoing many organic com- 

 pounds are absorbed in particular cases, as in those plants 

 which live in decaying animal or vegetable matter (sapro- 

 phytes), as well as those which absorb the juices from liv- 

 ing plants (parasites). 



133. Diffusion, — When absorbed, the solutions diffuse 

 through the watery protoplasm and the watery contents of 

 the vacuoles, " cell-sap." This diffusion continues from 

 cell to cell in thin-walled tissues, and is here known as 

 osmosis, the thin cell-walls serving as permeable mem- 

 branes through which the solutions pass. In laboratory 



