108 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



using cut leaves or shoots instead of growing plants in their entirety ; 

 but he gave no grounds for raising this objection. On the other hand, it 

 is easy to prove that all the functions of a leaf are carried on when 

 detached as when growing : transpiration can be readily detected ; and 

 M. Garreau found in his experiments on respiration that " detached leaves 

 gave the same results as those which remained attached to the plant ; " * 

 and if a green shoot be plunged into water the evolution of oxygen can 

 readily be seen. Moreover, M. Duchartre compared a shoot to a detached 

 limb of an animal, to which it is obviously not comparable ; for there is 

 no such mutual dependence between a shoot or a leaf and the main stem 

 as in the case of an animal's limb. The one can be detached and made 

 to strike root and grow into an independent plant ; not so the other. 



All that can be called injurious to a shoot, when detached for ex- 

 perimental purposes lasting for a short time only, is that the supply of 

 water is cut off. And I maintain, making due allowance for that fact, 

 whatever results a cut shoot or detached leaf gives in the matter of 

 absorption and transpiration are legitimately applicable to a growing 

 plant. Those who assert it to be otherwise must bear the burden of the 

 proof. 



M. Duchartre's experiments were made with plants growing in pots, 

 the latter being carefully protected from imbibing any moisture by a 

 mechanical contrivance. The plants thus prepared were weighed at 6 or 

 6.80 p.m., then exposed all night to dew. They were again weighed at 

 G or 6.30 a.m. on the following morning, w T ith the dew still upon them. 

 The leaves were then carefully wiped one by one till the whole plant was 

 dry. It was then again weighed ; and the result was that the weight was 

 almost exactly the same or more generally a little less than it was the 

 evening before. Duchartre consequently came to the conclusion that in 

 our climate dew is not absorbed directly by plants, but that it contributes 

 to their nutrition indirectly only, (1) by reducing the nocturnal transpira- 

 tion to nothing, and (2) by the intervention of the soil, which absorbs the 

 dew. 



The fundamental objection that I raise against his conclusion is that 

 he has not considered the difference that exists between the statical or 

 nearly statical conditions of the internal flow of water in a plant at night, 

 with the dynamical or active flow ever taking place as soon as transpira- 

 tion and evaporation are perfectly resumed in sunlight and heat. 



He has shown it to be true, though not so absolutely as has been 

 often asserted, that transpiration is greatly checked when the surfaces of 

 the transpiring organs are thoroughly wetted, or when in darkness. 

 Darkness and superficial moisture combined, as on a dewy night, must 

 therefore reduce this vital act to a minimum. The internal flow upwards 

 from the root, however, is not at the same time equally checked ; for the 

 temperature of the soil is not lowered to the same extent as that of 

 the air. 



Hence everything tends to bring the cells to as high a point of 

 tnrgidity at night as possible. Under these conditions one would hardly 

 expect aevi to b< imbibed in any appreciable quantity, unless the leaves 



* " '"' l« respiration die/, lcs plantes," Ann. des Sc. Nat. 3 mu s6r. xv. p. 12 (1851). 



