CH. v.] THE SAP. 181 



If tlie atmospheric temperature is so liigh that 

 moistm-e is emitted from the leaves faster than it 

 is suppUed, they droop or flag, and the growth of the 

 plant is suspended. If a cutting be placed in water, 

 it imbibes at first more rapidly than a rooted plant 

 of the same size, though this power rapidly decreases ; 

 but if planted in the earth, it at no time imbibes so 

 fast as the rooted plant, provided the soil is similarly 

 moist ; and this evidently because it has not such an 

 extensive imbibing surface as is possessed by the 

 rooted plant : consequently the soil in which a cutting 

 is placed, should be much more moist than is 

 beneficial to a rooted plant of the same species ; and 

 evaporation from the leaves should be checked by 

 covering the cuttings Avith a bell glass, or a Wardian 

 case would be still better. The temperature to 

 which the leaves are exposed should be approaching 

 the lowest the plant ^^ill endure. 



The wanner the soil within the range of tempera- 

 ture most suitable to the plant, the more active are 

 the roots, and the more energetically are caiTied on 

 all the processes of the vessels buried beneath the 

 surface of the soil: 50'^ for the atmosphere, and 

 between 65° and 75° for the bottom heat, are the most 

 effectual temperatures for the generality of plants. 



The cutting should be as short as possible con- 

 sistently ^^•ith the objects in \'iew. 



