THE PHYSICAL FUNCTIONS OF LEAVES. 365 



THE PHYSICAL FUXCTIOXS OF LEAVES. 



AN elaborate study on the above subject has lately been published 

 by Prof. J. Boussingault, of Paris, in the " Annales de Chimie et 

 de Physique " (vol. xiii., pp. 289-394), in which the phenomena of ab- 

 sorption and transpiration by leaves are treated at great length. Since 

 the memorable experiments of Hales in 1727, recounted in his work on 

 " Vegetable Statics," this branch of vegetable physiology has been 

 rarely touched, and the carefully recorded observations of Boussingault, 

 carried out with the best of modern scientific appliances, possess an un- 

 usual value. 



The first point studied was the loss of water by transpiration from 

 the leaves of plants under normal circumstances. For this purpose a 

 healthy Jerusalem artichoke [Helianthus tuherosus) in a roomy flower- 

 pot wa's chosen. The top of the pot was covered with a sheet of India- 

 rubber, tightly inclosing the stem of the plant, and provided with an 

 opening for the admission of water. The whole was then weighed, and 

 the loss noted which ensued under various circumstances, by evapora- 

 tion of water from the leaves, the plant receiving during the experiment 

 weighed normal amounts of water. The total surface of the leaves of 

 the plant (both upper and lower sides) was carefully estimated, and the 

 result reckoned on the square metre. The averages oL fourteen experi- 

 ments showed that the artichoke lost hourly, for every square metre of 

 foliage, the following amounts of water : in the sunshine sixty-five 

 grammes, in the shade eight grammes, during the night three grammes. 



In the next place the question was investigated whether the absorp- 

 tion of water by plants and the ascent of the sap are due to the force 

 resulting from the transpiration on the surface of the leaves, or whether 

 the roots exercise also a certain amount of force to this end. For this 

 purpose experiments similar to the above were carried out with various 

 plants, firstly under normal circumstances, secondly with the stem minus 

 the roots immersed in water. As an instance we can take mint. The 

 plant with roots showed an hourly evaporation per metre of eighty- 

 two grammes in the sunshine and thirty-six in the shade. Under the 

 same condition without roots, the evaporation Avas sixteen and fifteen 

 grammes respectively. 



The results show that the absorption of water by plants is deter- 

 mined in a great measure by the transpiration occurring in the leaves, 

 that this is maintained for a certain length of time without the assist- 

 ance of the roots, but cannot continue long, being dependent on the 

 injective power possessed by the roots. The efi'ect of pressure on the 

 absorption was next examined, and it was found possible by this means 

 for a time in certain cases to even more than replace the water lost by 

 transpiration. For example : a chestnut-branch dipped in water was 



