AIR AND LIGHT. 135 



265 It finally, in its altered state, sinks down the bark, whence it is given 

 off laterly by the medullary rays, and is disfributed throuj^h the system, 



266. No solid matter whatever can be taken ofF by the roots ; for this 

 reason, metals, which in the state of oxydes are poisonous, are perfectly 

 harmless in their metallic stite, as mercury ; and this is, no doubt, tho 

 cause why liquid manure, which contains all the soluble parts of manure 

 in a fluid state, acts with so much more energy than stimulating substances 

 in a solid state. 



267. Tfie cause of the motion of the sap is the attraction of the leaf- 

 buds and leaves. 



26S. The leaf-buds called into prowth by the combined action of tho 

 increasing temperature and light of spring, decompose tiieir carbonic acid 

 (279), and attract fluid from the tissue immediately below them ; the space 

 so caused is filled up by fluid again attracted from below, and thus a motion 

 gradually takes place in the sap from one extremity to the other. 



269. Consequently, the motion of the sap takes place first in the branches 

 and last in the roots. 



270. For this reason, a branch of a plant subjected to a high tempera 

 ture in winter, will grow while its stem is exposed to a very low tempera 

 ture. 



271. But growth under such circumetances will not belong maintamed^ 

 unless the roots are secured from the rpach of frost ; for, if frozen they 

 cannot act, and will consequently be unable to replace the sap of which the 

 stem is emptied by the attraction of the buds converted into branches, and 

 by the perspiration of the leaves (XII.) 



272. Whatever tends to inspissate the sap, such as a dry and heated 

 atmosphere, or an interruption of its rapid flow, or a great decomposition 

 of carbonic acid, by full exposure to light, has the property of causing ex- 

 cessive vigour to be diminished, and flower-buds to be ])roduced. 



273. While, on the other hand, whatever tends to dilute the sap, such 

 as a damp atmosphere, a free and uninterrupted circulation, or a great accu- 

 mulation of oxygen in consequence of the imperfect decomposition of car- 

 bonic acid, has the property of causing excessively rapid growth, and an 

 exclusive production of leaf-buds. 



274. Inspissaied or accumulated sap is, therefore, a great cause of fertility. 



275. And thin fluid, not being elaborated, is a great cause of sterility. 



276. The conversion of sap into diflTorent kinds of secretion is efTected 

 by the combined action of Air (.XL), Light (XL), and Temperature. 



XI. Air and Light. 



277. When an embryo plant (242) is formed within its integuments, it 

 IS usually colourless, or nearly so ; but, as soon as it begins to grow, that 

 part which approaches the liiiht (the stem) becomes coloured, while the 

 opposite extremity (the root) remains colourless. 



278. The parts exposed to the air absorb oxygen at night, absorb car- 

 bonic acid and part with oxygen again in daylight ; and thus in the day- 

 time purify the air, and render it fit for the respiration of man. 



279. The iiitensity of this latter phenomenon is in proportion to the 

 intensity of solar light to which leaves are directly exposed. 



2S0. It? cause is the decomposition of carbonic acid, the extrication of 



