292 ACTION OF LIGHT, ETC. 



with us the same point is also gained by cutting off the leaves, 

 or a part of them. 



But there is this disadvantage in cutting off the access of 

 light, that roots are formed more rapidly when cuttings are 

 exposed to light than when they are shaded, provided they can 

 be kept alive. It is therefore a great problem to determine 

 how much light a cutting will endure with impunity. The 

 power of bearing light varies from species to . species, and is 

 only to be determined by experience. One plant fades 

 presently, because its powers of perspiration are very great, as 

 is the case with the young shoots of most species of herbaceous 

 and shrubby plants ; but as they grow older the loss by 

 perspiration diminishes, because their thickened skin opposes 

 a mechanical obstacle, and they can bear more light. It would 

 therefore seem, at first sight, that ripened cuttings must in all 

 cases be preferable to those which are young and tender. 

 Certainly, they are less liable to_die_g[uickly ; but they are also 

 much more unwilling to root quickly. In fact, notwithstanding 

 the difficulty of keeping very young cuttings alive, they present 

 the only means of striking very difficult species, such, according 

 to Mr. Neumann, as the Cashew, the Mahogany, and the Litchi. 

 We may lay it down as a certain rule that the power of rooting 

 is always greatest in all cuttings when they are first pushing, 

 provided they have light. The misfortune is, that they are so 

 extremely perishable at that time. 



Water is our aid in this case. It is true that the sun's 

 influence can have no injurious tendency so long as the roots 

 can drink and the system digest as fast as the surface perspires ; 

 and that the reverse is fatal. But the whole surface of a plant 

 absorbs as well as evaporates, and the younger it is the more it 

 absorbs. It is therefore possible to give plants water by their 

 leaves ; and if this is done with skill, the bad influence of the 

 sun is prevented. In that case the cutting has time enough 

 to make roots, by which its grosser food may be conveyed to it. 

 Hence has arisen the practice of striking plants under bell- 

 glasses, fitting tight to the soU on which they rest. Ignorant • 

 people believe that the use of a bell-glass is to keep out air, 

 which is impracticable and useless. Bell-glasses act by keeping 



