sap. 165 



266 No solid matter whatever can be'taken up by the roots ; 

 for this reason metals, which in the state of oxydes are poi- 

 sonous, are perfectly harmless in their metallic state, as mer- 

 cury ; and this is, no doubt, the 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. The cause of the motion of the sap is the attraction of 

 the leaf-buds and leaves. 



268. The leaf-buds called into growth by the combined ac- 

 tion of the increasing temperature and light of Spring, decom- 

 pose their carbonic acid (279.), and attract fluid from the tis- 

 sue 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 

 temperature in Winter, will grow while its stem is exposed 

 to a very low temperature. 



271. But growth under such circumstances will not be long 

 maintained, unless the roots are secured from the reach 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 c inverted 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 excessive vigour to be 

 diminished, and /lower-buds to be produced. 



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 accumulation of oxygen in consequence 

 of the imperfect decomposition of carbonic acid, has the pro- 

 perty of causing excessively rapid growth, and an exclusive 

 production of leaf-buds. 



274. Inspissated or accumulated sap is, therefore, a great 

 cause of fertility. 



275. And thin fluid, not being elaborated, fs a great cause 

 of sterility. 



276 The conversion of sap into different kinds of secretion 

 is effected by the combined action of Air (XI.), Light (XI.), and 

 Temperature. 



XI. Air aivd Light. 



277. When an embryo plant (242.) is formed within its in- 

 teguments, it is usually colourless., or nearly so ; but, as soon 



