FLOWER-GARDENIXG. 151 



267. The cause of tlie motion of the sap is the attraction of 

 the leaf-buds and leaves. 



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

 action of the increasing temperature and light of spring, de- 

 compose their 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 temperature in winter will grow w^hile its stem is ex- 

 posed 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 converted into branches, and by the 

 perspiration of the leaves (XII.). 



272. Wliatever 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 vigor to be dimi- 

 nished, and flower-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 circu- 

 lation, or a great accumulation of oxygen in consequence of the 

 imperfect decomposition of carbonic acid, has the property 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, is a great cause of 

 sterility. 



276. The conversion of sap into diftcrent kinds of secretion 



