TRANSPIRATION. IO5 



The following is from Sachs : 



A branch of White Poplar, weighing 125-2 grms., with a leaf-surface 

 of 2700 sq. centim., lost 480 c.c. of water in no hours ; hence from every 

 square centimetre there evaporated a column of water of r8 mm. in 

 height ; from each square centimetre of the surface of the water in an 

 adjacent vessel there evaporated, in the same time, a column of water 

 5 mm. in height. In an experiment with Heliantkus annuus it was found 

 that the loss from each square centimetre of leaf-surface was 2^23 mm., 

 and from each square centimetre of water-surface, 5-3 mm. Sachs points 

 out that this proportion is probably too large, for the external surface of 

 a leaf by no means represents the whole surface from which transpiration 

 is taking place ; the intercellular spaces must be taken into account. 

 He assumes, therefore (and this is probably below the mark), that the 

 transpiring surface of the leaves in these experiments may be taken to be 

 ten times greater than their external surface. The proportion between 

 the amount of water evaporated from the leaves and from the vessel will 

 therefore be in the case of the Poplar, and $ in the case of the Sun- 

 flower. 



The explanation of this fact is doubtless this, that the 

 living cells offer a considerable resistance to the evaporation 

 of the water which saturates them. This is not the case with 

 a dead membrane; on the contrary, the experiments of Sachs 

 shew that water evaporates more readily from a membrane 

 than from a free surface, and it has been frequently observed 

 that parts of plants which have been killed dry up very 

 rapidly. 



It appears that, cateris paribus,ft\z transpiration is pro- 

 portional to the surface of a leaf, though, as might be expected, 

 the activity of transpiration is very different in the leaves of 

 different structure : thin herbaceous leaves, for instance, tran- 

 spire much more freely than do those which are fleshy or 

 coriaceous. Further, it was pointed out by Guettard, and his 

 observations have been confirmed by those of Bonnet, Unger, 

 and Garreau, that transpiration takes place more actively 

 from' the lower than from the upper surface of the same leaf, 

 a fact which must be attributed to the greater number of 

 stomata on the under surface. 



