148 NOTES AND MEMORANDA. 



4tli. The temperature of the soil has much less influence on 

 absorption than that of the air (by the intermediation of transpiration) 

 under ordinary conditions of moisture. For a much stronger reason, 

 it is of but little consequence for a plant growing in the open air to 

 be exposed to the burning rays of the sun. 



The same author follows this paper by one " On the direct Com- 

 jiarison of Absorption with Transpiration." * The principal conclu- 

 sion to which a long and careful series of experiments has led him, is 

 that the amount of absorption does not bear any direct proportion to 

 that of transpiration ; and his general results are summed up as 

 follows : — 



1. Of all the theories proposed up to the present to explain the 

 motion of the water in the plant, that of Boehm (referred to hereafter) 

 is most in harmony with observed facts. 



2. Although transpiration is the most powerful cause of absorp- 

 tion, these two functions are not necessarily proportional. 



3. Absorption is equal (in general terms) to transj)iration when 

 the i^lant grows in average and very slightly varying conditions ; for 

 example, in diffused light and in moderately moist air. 



4. When a plant removed from these average conditions is ex- 

 posed to dry air, transpiration is much stronger than absorption. It 

 cannot possibly attain so high a figure as transpiration ; the plant 

 withers, and it is exjjosed to an irreparable disturbance, which consists 

 perhaps in the abnormal destruction of the vacuum existing in the 

 plant. 



5. When a plant is removed from these average conditions of 

 growth, and exposed to air satiirated with moisture, the absorption, 

 obeying the already existing vacuum, is stronger than the transpira- 

 tion ; but in proportion as the vacuum fills up, the absorption decreases, 

 and finally ceases if the transpiration has also ceased (state of re- 

 pletion). 



6. When a plant lacks water, the suction produced by transpi- 

 ration is not lost ; it accumulates, and comes into operation as soon 

 as the roots come in contact with water. An absorption much more 

 energetic than the transpiration is then observed ; but this continues 

 to diminish in ]3roportion as the existing vacuum fills, to be finally 

 regulated by the intensity of transpiration. 



The paper by M. Boehm, " On the Causes of the Ascent of Sap," f 

 is a very valuable one : — 



Even now, he says, the majority of physiologists consider the 

 movement of the water excited by transpiration in the turgescent 

 cells of the leaves to be a purely osmotic phenomenon. Owing to the 

 continuous production of organic matter in the assimilating cells, the 

 osmotic tension would always have such an intensity that the water 

 coming from the neighbouring cells would replace the loss caused by 

 transpiration. This view is, however, as he believes, erroneous, for 

 the following reasons: — 



1st. The movement of water produced by osmose is extremely slow. 



* ' Ann. des Sci. Nat.' (Bot.), 3rd ser., vol. vi. (1S7S) p. 201. 

 t Ibid., p. 223. 



