PERSPIRATION OF LEAVES, 155 



vive, by which their power of absorption is also proved. 

 Hence the use of a tin box to travelling botanists, for 

 the purpose of restraining the evaporation of plants, and 

 so preserving them fresh for some days till they can be 

 examined, as well as of reviving faded plants, if the in- 

 side of the box be moistened before they are shut up 



in it. 



Dr. Hales found that a plant of the Great Annual Sun- 

 flower, Helianthus annuus, lost 1 lb. 14 oz. weight in 

 the course of twelve hours in a hot dry day. In a dry 

 night it lost about 3 oz.; in a moist night scarcely any 

 alteration was observable, but in a rainy night it gained 

 2 or 3 oz. The surface of the plant compared with 

 that of its roots was, as nearly as could be calculated, in 

 the proportion of five to two ; therefore the roots must 

 have imbibed moisture from the earth of the pot in which 

 the plant grew, and which was all previously weighed, 

 in the same proportion of five to two, otherwise the 

 leaves would have faded. The same experiment was 

 made on the Vine, the Cabbage, &c., with various re- 

 sults as to the exact degree of perspiration, but all prov- 

 ing it to be considerable. Evergreens arc found to 

 perspire much less than other shrubs. 



The state of the atmosphere has a great effect on the 

 rapidity of this perspiration. Practical botanists know 

 how much sooner plants fade, and haymakers experi- 

 ence how much faster their work is done, some days 

 than others, and those days are by no means always the 

 most sunny. In a hot dry day plants are often exhaust- 

 ed, so as to droop very much towards evening, especial- 

 ly in the dry unsheltered bed of a garden. Such as have 



