Dixon — Transpiration into a Saturated Atmosphere. 631 



not appear to be restricted to any special cells forming water-glands 

 on the surface of the leaf. It seems most probable that most or all of 

 the cells bordering on the vascular capillaries, both in leaf and stem, 

 are able to exert a tractional force on the water in the conduits, and 

 are able to expel water, when thus drawn in, on theii* outer surfaces. 

 It may be, however, that the cells of the water- glands of plants are 

 more highly specialized for this function, and hence the exudation of 

 drops on leaves of plants in moist atmosphere takes place over these 

 glands or hydathodes, as Haberlandt prefers to call them. 



The following observation shows that the elevation of the water 

 is not solely due to the functioning of these water-glands, even in 

 plants possessed with these structures. The leaves of JEscallonia ma- 

 crantha, Chrysanthemum sinense, and Chrysanthemitm lacustre have water 

 stomata on the margin of the leaf ; but if these glands are removed by 

 cutting away the whole margin with a scissors, it will be found that 

 water will be drawn up into these leaves through the stem almost 

 as quickly as into the leaves of a branch which are left intact. 



Another observation which shows that the traction is exerted by 

 cells of the leaf, which are not visibly di:ffierentiated, may be made 

 on Cheiranthus Cheiri. The leaves of this plant, so far as I can make 

 out, have no specialized water-glands. However the extreme apex 

 often withers away in the older leaves, as if some substance had been 

 exuded there fi'om the leaf. In case this tip be the seat of a water- 

 gland, it is removed from all the leaves of a branch which is set in 

 the saturated chamber. Alter a suitable time it will be found that 

 the coloured fluid has risen into all the veins of the leaves, and it 

 will be seen in the ultimate blind terminations of the vascular bundles. 

 In Cheiranthus Cheiri these terminations are surrounded by cells un- 

 differentiated from the other cells of the mesophyll of the leaf. The 

 coloured fluid must have been drawn into the terminal portions of the 

 veins by these cells, and not by any specialized water-glands. We 

 may conclude that the similar cells along the conduits have the same 

 function. 



Of course it is quite possible that the cells forming the water- 

 secreting tissue of the water-glands have this pumping power more 

 highly developed than that possessed by the cells situated all along 

 the conduits. Hence, in Gardiner's^ experiments, the expulsion of 

 water occurred in noticeable quantities only over the water-glands. 

 He found that drops of water are exuded fi'om the water-glands of 



1 Proc. Cambridge Phil. Soc, vol. v. 1SS4. 



