ORGANS OF CIRCULATION. 95 



fluid will in all cases pass through a thin membrane 

 into a thicker fluid : and in all such cases there will 

 be, according to circumstances, an inflow 1 , or an 

 outflow 2 , depending on electrical agency. It is upon 

 this principle that he thinks the spongelets in the 

 root, containing a fluid thicker than the carbonised 

 water in the soil around them, imbibe it and force it 

 up to the tops of the highest trees, 



Hales cut off in the spring the branch of a vine, 

 and enclosed the cut surface of the stump in a bent 

 tube, when the sap flowed so abundantly and with 

 such force, as to sustain thirty-eight inches of mer- 

 cury, equal in weight to a column of water forty- 

 three feet high, which indicates a force five times 

 greater than the current of the blood in a horse. The 

 fresh cut branch of a vigorous pear-tree, he found, 

 raised the mercury eight inches in six minutes. 

 Bonnet, in experimenting upon blanched plants with 

 coloured fluids, found that these rose within the plants 

 from two to three inches in an hour. 



That heat *is a main agent in raising the sap is 

 proved by the experiment of placing in two similar 

 vases of water, two similar vine plants, leaving the 

 stem of one in the open air, and introducing the stem 

 of the other through the glass into a hot-house: in 

 the latter the buds will soon expand, and the water 

 become rapidly exhausted; while in the other, the 

 buds will swell slowly, and the water will be slowly 

 diminished. 



During the day also, and particularly when the 



(1) In French Greek, Endosmose. 



(2) In French Greek, Ezosmose. 



