204 APPLICATION OF PRINCIPLES. 



and to assist in the formation of roots ; but this is a 

 very delicate operation, for, if the leaf is allowed 

 to suffer by excessive perspiration, the cuttings must 

 necessarily perish (75). To maintain a steady saturat- 

 ed atmosphere around a cutting stops this 

 danger, and hence the' use of a bell glass. A 

 double glass has been lately recommen ded 

 {fig. 21) ; but if this precaution is of any 

 value, it must be, not because it main- 

 tains an even temperature, which is 

 injurious rather than useful, but because it prevents 

 condensation upon the inner bell glass, and the con- 

 sequent abstraction of atmospheric moisture, and pro- 

 bably acts at the same time as a kind of shade. 



Notwithstanding the precaution of covering cut- 

 tings with a bell glass, shade is also necessary, as a 

 further security against perspiration ; for light acts as 

 a specific stimulus (71), whose effects are very dim- 

 cult to counteract. It must, however, be employed 

 with great caution ; for, if there is not light enough, 

 the leaves attached to the cuttings cannot form that 

 organisable matter out of which roots are produced. 



All gardeners know that the root end of a cutting 

 should be dose bebw a leaf-bud; this is to facilitate 

 the emission of roots by the buds, which emission 

 must necessarily take place with greater or less diffi- 

 culty in proportion as their exil is facilitated or imped- 

 ed by the pressure of bark on them.* 



* [The amateur will find it advantageous to take off the cutting, 

 when delicate or short, close up to the main stem, so as to preserve 

 the collar or enlarged portion of wood at the base of the branch. 



