1898] NOTES AND COMMENTS 153 



pare the day and night position of the leaves of clover, Canadian bean, 

 or Rohinia, we see that the result, effected by somewhat different move- 

 ments, is the same, namely, to expose as little as possible horizontal 

 leaf-surface. In the bean and Rohinia the leaflets hang more or less 

 vertically, in clover they are arranged in a neat little packet. The 

 view taken by Darwin, and generally held since, is that the object of the 

 movement is to prevent an excessive radiation of heat. The leaf is 

 necessarily a delicate structure, functioning as the seat of the most 

 important life-processes of the plant, and it is easy to conceive the 

 danger of damage from excessive loss of heat by radiation to a 

 clear sky if the leaf retained at night its horizontal day position. 



In a recent paper {Botanischc Zcitwiig, vol. Iv., 1897, p. 71), Stahl 

 criticises this view, and suggests that leaf-movements are associated with 

 the giving up of water- vapour (transpiration). The object of tran- 

 spiration is the maintenance of a current of water carrying in solution 

 food-stuff absorbed from the soil by the root up to the leaves, and 

 the sleep-movement, far from being a sign of repose, is an adaptation 

 for enabling this process to go on by night as well as by day. In 

 most plants the apertures from which the water-vapour escapes, or 

 stomata, are closed all night and transpiration is reduced to a mini- 

 mum. Stahl finds, however, that in cases where the leaves show the 

 nyctitropic movement the stomata remain open. The object of the 

 vertical position seems to be to prevent deposition of dew on the leaf, 

 and merely to remove an effective obstacle to transpiration of water- 

 vapour. In some plants the night position is also assumed in very 

 strong sunlight. There the action has the opposite effect, namely, to 

 check transpiration and remove the danger arising from excessive loss 

 of moisture. It recalls the permanent vertical position of the leaves 

 of many Australian Mimosas, Eucalyptus and Hakeas, which are 

 exposed to intense sunlight. Stahl suggests that the spontaneous up 

 and down movements of the leaflets of the American telegraph-plant 

 {Desmodium gyrans), are to promote transpiration, the motion caus- 

 ing a continual removal of the saturated air near the leaf, and the 

 shivering of the leaves of the aspen and many poplars may have the 



same meaning. 



TUKGIDITY 



In connection with the amount of water in plants, the experiments of 

 Mr E. B. Copland on " the relation of nutrient salts to turgor," described 

 in the December number of the Botanical Gazette, are of interest. 

 Since the work of Pfeffer and others on the hydrostatics of the plant- 

 cell, and the demonstration of the great internal pressure set up by 

 osmosis, the turgidity of the cell has been an important factor in the 

 explanation of plant-growth and movement. As in the case of other 

 newly-discovered causes, it was at first made responsible for too 



