DIFFUSION OF FLUIDS. 569 



passes off almost as pure vapour, and, at all events, leaves its 

 mineral constituents behind. The amount of evaporation is re- 

 markably great, and accounts in some degree for the sustenance of 

 plants by such extremely dilute solutions of their nutrient matters 

 as they find in the soil. 



Amount of Transpiration. The experiments of Lawes and Gilbert 

 give the following average daily loss of water in grains in the months 

 indicated, in pots of unmanured soil, the first line from Wheat, the 

 second from Peas : 



March 19th March 28th April 28th May 25th June 28th July 28th Aug. llth 



to to to to to to to 



March 28th. April 28th. May 25th. June 28th. July 28th. Aug. llth. Sept. 7th. 



14-3 40-9 162-4 1177-4 1585-3 1101-4 230-9 

 11-2 42-9 106-4 1079-8 2092-7 377-2 



The total amount of water given off during the whole period of 172 

 days (March 19 to Sept. 7) was, by the Wheat, 113,527 grains, by the 

 Peas, 109,082 grains. The total quantity of mineral ash from each of the 

 samples was, Wheat, 36-49 grains, and Peas, 43-16 grains, which shows 

 that the Wheat took up 32-14 grains and the Peas 39-57 grains of mineral 

 matter in every 100,000 grains of water which evaporated from it. 



Other elaborate experiments of the same observers, recorded in the 

 Journal of the Horticultural Society for 1851, show that evergreen trees 

 transpire less than deciduous trees, and that great differences are manifested 

 in different plants according to temperature. The maximum of evapora- 

 tion does not always coincide with the maximum temperature. It was 

 also found that plants cultivated without manure frequently evaporated 

 more than those to which manure was applied ; and, further, that 

 under a purely mineral manure more water was transpired than when a 

 mixed mineral and ammoniacal manure was used, so that the more abun- 

 dant the food the less water transpired. Deherain shows that, as a rule, 

 the amount transpired is greater from the younger than the older leaves. 

 Haberlandt, in his experiments on this subject, shows that young grow- 

 ing Cereals before the period of flowering transpire most, and least of all 

 after that process, the relative proportion of root or absorbing organs as 

 compared to transpiring organs being then greatest. The four plants 

 examined by M. Haberlandt evaporated in 24 hours per 100 square centi- 

 metres during the whole period of vegetation (90 days) as follows : 

 Barley 3794, Wheat 3532, Rye 2849, Oats 2666 grammes. But con- 

 sidered in relation to the surface of the plants, the Oat evaporated 2277, 

 the Barley 1236, the Wheat 1179, and the Eye 834 grammes per plant 

 during the same period. Supposing a million plants on a hectare (a 

 hectare =2| acres about), the loss of water, according to the above calcu- 

 lations, over that amount of surface would be : Rve 83,490, Wheat 

 1,179,920, Barley 1,236,710, and Oats 2,277,760 kilogrammes, which 

 corresponds to a rainfall respectively of 83-5 millimetres, 118 millims., 

 123-7 millims., and 227*8 millims. In all cases great variations, both as 

 to absorption and transpiration, occur in different individuals of the same 

 species. 



