CUTTINGS. 169 



§17. As it is the action of the leaf buds that causes growth 

 in a cutting, it follows that no cutting without a leaf-bud will 

 grow ; 



318 Unless the cntting has great vitality and power of form- 

 ing adventitious leaf-buds (119.), which sometimes happens. 



319. An eye is a leaf-bud without an internodium. 



320. It only differs from a cutting in having no reservoir of 

 food on which to exist, and in emitting its roots immediately 

 from the base of the leaf-bud into the soil. 



321. As cuttings will very often, if net always, develope 

 leaves before any powerful connection is formed between 

 them and the soil, they are peculiarly liable to suffer from 

 perspiration. 



322. Hence the importance of maintaining their atmosphere 

 in an uniform state of humidity, as is effected by putting bell 

 or other glasses over them. 



323. In this case, however, it is necessary that if air-tight 

 covers are employed, such as bell-glasses, they should be from 

 time to time removed and replaced, for the sake of getting 

 rid of excessive humidity. 



324. Layers differ from cuttings in nothing except that they 

 strike root into the soil while yet adhering to the parent plant. 



325. Whatever is true of cuttings is true of layers, except 

 that the latter are not liable to suffer by evaporation, because 

 of their communication with the parent plant. 



326. As cuttings strike roots into the earth by the action of 

 leaves or leaf- buds, it might be supposed that they will strike 

 most readily when the leaves or leaf-buds are in their greatest 

 vigour. 



327. Nevertheless, this power is controlled so much by the 

 peculiar vital powers of different species, and by secondary 

 considerations, that it is impossible to say that this is an ab- 

 solute rule. 



328. Thus Dahlias and other herbaceous plants will strike 

 root freely when cuttings are very young; and Heaths, Aza- 

 leas, and other hard wooded plants, only when the wood has 

 just begun to harden. 



329. The former is, probably, owing to some specific vital 

 excitability, the force of which we cannot appreciate; the 

 latter either to a kind of torpor, which seems to seize such 

 plants when their tissue is once emptied of fluid, or to a natu- 

 ral slowness to send downwards woody matter, whether for 

 wood or not, which is the real cause of their wood being harder. 



330. If ripened cuttings are upon the whole the most fitted 

 for multiplication, it is because their tissue is less absorbent 

 than when younger, and that they are less likely to suffer 

 either from repletion or evaporation. 



331. For to gorge tissue with food, before leaves are in action 

 to decompose and assimilate it, is as prejudical as to empty 

 tissue by the action of leaves, before spongioles are prepared 

 to replenish it. 15 



