ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 489 



absorbed in adult leaves in darkness is a constant quantity for the 

 same species. The increase in the intensity of respiration with a 

 rise of temperature may be represented by a parabolic curve. 



Movement of Ascending Sap.* — M. J. Vesque has paid special 

 attention to two points in this question, viz. the influence of external 

 pressure on the absorption of water by the roots ; and the part played 

 in this movement by the vessels. 



On the first point he comes to the following general conclusions : — 

 In the case of the oleander the absorption of water by the roots 

 depends on the external pressure, and appears to augment in pro- 

 portion to the difference between this pressure and that of the aii* 

 contained in the woody substance of the root. The pressure of the 

 internal air depends on transpiration and osmose; the latter force 

 does not appear to be always active ; absorption may be arrested by 

 decreasing the atmospheric pressure. Under the conditions employed 

 the pressure of the internal air did not differ greatly from the atmo- 

 spheric pressure ; it was ordinarily somewhat less ; in only one case 

 was it observed to be slightly greater. The effect of external pressure 

 on the oleander is sufficiently great for a rapid movement of the 

 barometer to produce a sensible disturbance in the absorption of 

 water by the roots. The bean, and possibly all herbaceous plants, 

 are much less influenced by external pressure than woody plants in 

 reference to the absorption of water. The influence does, however, 

 exist, although usually unperceived by the side of fluctuations induced 

 by variations in transpiration and by other secondary causes. The 

 rapidity of the movement of water in the plant should be greater in 

 proportion to the difference between the pressure of the air imprisoned 

 in the upper and in the lower part of the plant. External pressure, 

 as well as osmose, may increase this difference. 



On the second point, the following are the conclusions of the 

 author: — Water is in motion in the young vessels whenever it is 

 possible ; i. e. whenever they are not obstructed by accident or by 

 strings of bubbles of air. Whenever either of these is the case, lateral 

 passages are established in consequence of the rapid diminution of 

 the pressure of the air contained in the adjacent woody cells, and 

 the water passes from the vessels into these cells, whence it escapes 

 by the operation of differences of pressure in the woody cells. This 

 lateral passage of water is so rapid that a current is formed in 

 them beneath any obstacle; but this phenomenon does not differ 

 essentially from the normal condition, since the vessels are closed at 

 their extremity. When the open extremity of the vessels of a cut 

 branch or of a leaf taken from a herbaceous or woody plant is stopped, 

 these branches or leaves wither and dry up. When a branch is bruised 

 for a length of some millimetres, so as to close the cavities of the vessels 

 and tracheids, the branch will wither. The author believes that the 

 water placed in movement in the passage from one cell to another is 

 water of imbibition, and considers the term filtration to be improperly 

 used in this connection. 



* Ann. Sci. Nat.— Bot., xix. (1881) pp. 159-99. 



