FOOD AND SECRETIONS. 33 



It has, however, been proved experimentally that they purify the air much more 

 by their assimilating (254) action, than they vitiate it by their respiration. 



257. No plants can long exist in which this alternate action 

 is prevented, unless, perhaps, Fungi and brown parasites. 



258. The amount of assimilation is determined by the de- 

 gree of light to which a plant is exposed. It is light alone 

 that causes, in conjunction with vital forces, the decomposition 

 of the matters contained in living plants. 



259. Hence, if a plant is compelled to grow in darkness, no 

 assimilation takes place of the food that the roots receive ; 

 oxygen accumulates ; its natural proportion to other elements 

 is disarranged ; and a destruction of the tissue takes place. 



260. In order to avoid this, plants will always lengthen 

 themselves in the direction in which the smallest ray of light 

 approaches them, as is the case of seed which shoot from dark- 

 ness into light. If this is impossible, they become blanched or 

 etiolated, and then die. 



261. From the continued assimilation of the elementary 

 constituents of plants, new products result, and serve for the 

 formation of woody fibre, and all solid matters of a similar 

 composition. The leaves produce sugar, starch, and acids, 

 which were preyiously formed by roots, when necessary for the 

 developement of the stem, buds, leaves, and branches. 



Some phyto-chemists believe that during the chemical transformations that result 

 in plants from the separation and re-combination of their elements, two com- 

 pounds are necessarily formed, one of which remains as a component part, 

 while the other is separated by the roots, in the form of excrementitious mat- 

 ter. But the experiments upon which this supposition is founded are not con- 

 sidered conclusive ; and great doubt is entertained whether plants have really 

 the power of rejecting excrementitious matter by their roots. It appears more 

 probable that the necessary separation of effete matter takes place by the 

 hairs and glands that clothe the surface of plants, or by a fluid secretion from 

 their whole surface. 



262. Sap (251) is put in motion by the newly developing 

 leaf-buds, which, by constantly consuming the sap that is near 

 them, attract it upwards from the roots as it is required. 

 Therefore, the movement of the sap is the effect, and not the 

 cause, of the growth of plants. It depends upon vital irritabi- 

 lity, and is independent of mechanical causes. 



263. This irritability is indicated not only by the motion 

 of the sap, but by several other phenomena of vegetation ; 

 such as, 



The elasticity with which the stamens sometimes spring up when touched, and 

 the sudden collapse of many leaves when stimulated ; the apparently spon- 



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