24 VEGETABLE PHYSIOLOGY. 



of the syphon, and remained stationary at the height of thirty-eigUt 

 inches above its original level. This column of mercury, it is 

 obvious, represents a pressure very much greater than that of oar 

 atmosphere. 



The ascent of the sap in trees takes place by the woody layers. 

 It is easy to obtain conviction of this by making a plant absorb a 

 watery solution of cochineal. By making sections in the stem at 

 different heights, we can readily trace the colored liquid in its pro- 

 gress ; it is undoubtedly the course which the natural sap would 

 nave taken. We see no indication of the coloring matter in the pith 

 nor in the bark, the woody tissue alone is colored, sometimes en- 

 tirely, but more generally in its younger parts only. The dyeing 

 which results from this injection of the wood is in lines, and parallel 

 with the axis of the trunk, like the woody fibres themselves ; but 

 in some cases the sap may deviate from the rectilinear course. 

 Hales showed this by the following experiment : upon a tree he 

 made four notches, one above the other ; each notch occupied one 

 quarter of the trunk and reached to its centre. In this way the 

 whole of the woody fibres were cut through at different heights, so 

 that to continue its ascent the sap must necessarily experience a 

 series of lateral deviation, which in fact took place. 



The ascending sap of vegetables, as it has hitherto been procured 

 for examination, is an extremely watery fluid, holding in solution 

 very small quantities of divers saline and organic substances. 

 Having attained the leaves, the sap there undergoes modification, 

 and becomes concentrated by losing water. It at the same time 

 experiences, through the agency of the atmospheric air, under the 

 influence of light, a great modification in its constitution. Thus 

 elaborated, the sap takes a descending course ; following the liher^ 

 it retrogrades towards the soil, and therefore performs a kind of cir- 

 culation in its passage through the plant. The descending course 

 of the sap is demonstrated by throwing a ligature round the trunk 

 of a tree ; after a time there is formed, above the part that is tied, 

 an enlargement which is owing to the accumulation of the principles 

 of the sap ; but below it the tree experiences no increase. The 

 descending course of the elaborated sap is no effect of simple gravi- 

 ty ; because, if the ligature be thrown around a pendent branch, the 

 enlargement still forms between the ligature and the free extremity 

 of the branch. The descending sap passing through the cortical 

 layers must necessarily contribute to their formation ; and it is 

 almost certain, as appears from the capital experiment of Duhamel, 

 that it is the cambium which is changed into liber, and so concurs in 

 the growth of trees. The concentration of the ascending sap, which 

 occurs during its passage through the leaves, by the simple effect 

 of evaporation, is the phenomenon which is spoken of under the 

 name of the exhalation of plants : this exhalation of plants, it is 

 easily understood, is favored by a high temperature, dryness, and 

 motion of the air. In favorable circumstances, the water escapes 

 in the state of vapor. Hales compared the watery exhalation of 

 Dlants to the perspiration of animals, and made many experiments 



