CH. TI. 



COURSE OF TITE SAP. 



45 



wood, an cargumeiit that the leaf is the organ of excre- 

 tion ? The quantity of ashes or inorganic matter left 

 by the leaves when burnt is perhaps from twelve hun- 

 dred to two thousand per cent, greater than is left by 

 the wood ; that is, it is from twelve to twenty times as 

 much. And it is possibly on this account that these 

 organs of excretion are themselves excreted. The 

 division of trees into deciduous and non-deciduous is 

 not strictly correct. All are deciduous ; that is, all de- 

 foliate or lose their leaves — those which we call 

 deciduous generally in about six months, those which 

 we call evergreens generally in about twelve months. 

 Evergreen trees, however, differ in time, but each has 

 its fixed period for defohation. 



Certainly the transpiration or giving off of water 

 from the leaf, when exposed to drought, is very rapid, 

 and the communication from the root to the leaf very 

 rapid and constant ; since on felling trees of thirty or 

 forty feet high, while they were shooting in the summer, 

 I have observed the shoots lose their turgescence ; that 

 is, droop, in the course of a few minutes from the time 

 that the stem is divided from the root. The accurate 

 and admirable Hales found that a sunflower, in dry, 

 hot w^eather, gave off two pounds and a half, that is, 

 two pints and a half, of water in twelve hours. At 

 night, and in moist weather, the quantity was much 

 less. Senebier supposes that plants give off two-thirds 

 of the water which they absorb. 



Many physiologists imagine that the great use of the pogg^^J^P^ 



