GROWTH OF PLANTS. 61 



effected by the assistance of the cam'bium. One condition necessary to 

 the success of the operation is, that the sap of the two plants shall be 

 similar ; for example, the plants of the same genus, or of the same family, 

 are more readily grafted upon each other than those which belong to dif- 

 ferent families. Grafting is a very useful operation in agriculture ; it 

 serves to preserve and multiply varieties which could not be produced by 

 means of seeds ; it saves time by procuring a great number of trees which 

 are with difficulty multiplied by other means, and accelerates by many 

 years the fructification of certain plants. 



Gardeners employ five or six different processes to obtain the develop, 

 ment of the bud or graft upon the bark of other trees which they use as 

 stocks. 



Splice or whip grafting, consists in paring down in a slanting direction 

 both the graft and stock, and, after applying them neatly to each other, 

 securing them by strands of bast matting, in the same manner as two 

 pieces of rod are spliced together to form a whip handle. The part is after- 

 wards covered with tempered clay, or any convenient composition that will 

 exclude the air. 



Grafting by approach, or inarching, is a mode of grafting in which, to 

 make sure of success, the graft or scion is not separated from the parent 

 plant until it has become united to the stock.] 



70. Such are the principal phenomena of the life of nutrition 

 in plants : but they are far from taking place with the same in- 

 tensity at all times ; and their duration is extremely variable. 



71. In every plant we observe periods of activity, of languor, 

 and even torpor, and then an augmentation of the vegetative 

 functions. In our climate these periods correspond with the four 

 seasons of the year. During winter, the cold and absence of the 

 leaves, in most plants, almost entirely arrests nutrition ; they are 

 then in a state of torpor, comparable to that which hibernating 

 animals experience, and their buds and roots alone continue to 

 grow. But when returning spring imparts to the plant thus 

 benumbed a certain amount of heat and moisture, it awakes in a 

 measure, the sap rises with force, the buds develope themselves, 

 the young shoots or scions become elongated, and vegetation 

 displays all its activity. In summer the leaves are somewhat 

 hardened, and become less suited for attracting the sap and 

 exhaling the liquids which reach them from the roots ; conse- 

 quently vegetation is less active: and in autumn this change in 

 the leaves being greater, gradually brings about their destruction 

 or fall. At this period, it sometimes happens that buds begin to 

 develope themselves, and again attract the sap with force ; and 

 this ascent of the nutritive juices causes an elongation of the 

 branches and the formation of new leaves, the freshness of which 

 is in beautiful contrast with the yellow tint of the old ones. But 

 the cold soon enfeebles all these phenomena of life, arid arrests 



70. Is the duration of all plants the same ? 



71. Are the functions of vegetables always equally active ? How is their 

 activity influenced ? 



6 



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