90 M. BIOT ON THE SAP OF PLANTS. 



and facile. The diminution of the exhaling- power by a sudden cold 

 will favour it, and the sap will burst forth at once from all the pores 

 of the tree equally, taking into account merely the different degrees of 

 thickness in the bark. Such is an account of the emission of sap by 

 the sides of the nut tree and sycamore, in spring. 



The influence of the leaves on the internal motions of the sap in 

 trees being thus explained, let us observe what will be the consequence> 

 if these leaves, or great evaporating organs, be enveloped with a colder 

 atmosphere. The sap conveyed to them being no longer evaporated, 

 will rest and collect on their surface, and check all evaporation, espe- 

 cially at night. The upper parts of the vegetable tissue, or hygro- 

 scopic column, being thus overcharged, will let fall their superabun- 

 dance upon the parts that are lowest, which will produce a descent of 

 the sap. Hence proceed the alternate ascent and descent of sap, such 

 as have been noticed. Moreover, these effects will become continuous, 

 if the evaporating property of the leaves should diminish before the 

 supplying power of the root ceases to throw up the sap ; and this is 

 precisely the case in September : the same trees that afforded me but 

 their ascending sap in the spring, in September afforded a continual 

 sweat. The latter was no longer the same as the spring sap, for it 

 contained no saccharine principle. 



M. Biot concludes from his experiments, that the alimentation of 

 the foliaceous organs is accomplished principally during the day ; whilst 

 the alimentation of roots, and the formation of new layers of them, is 

 effected during the night, when the diminution of evaporating power 

 in the leaves precipitates the sap in a descending course towards the 

 roots. 



That in deciduous trees, the annual increase of the trunk and branches 

 taking place in summer, the increase of the roots takes place in winter. 

 The ascending motion is thus suspended by the cold, and the absence 

 of leaves allows the sap to accumulate in the roots, which experience 

 little of the atmospheric variations, and which, in the first warmth of 

 spring, send up their accumulated juices with force through the upper- 

 most parts of the tree. 



