STRUCTURE.] OEIGIN OF LEAF-BUDS. 173 



the leaf. This is undoubtedly an universal rule ; buds whether 

 normal, latent, or adventitious, being invariably formed by 

 exciting the peculiar vitality of true cells, and not of tubular 

 forms of tissue ; a very important physiological truth. 



We are unacquainted with the cause of the formation of 

 leaf-buds ; all we know is, that they proceed exclusively from 

 cellular tissue ; and if produced on the axis, from the mouths 

 of medullary rays. It would seem as if certain unknown forces 

 were occasionally so exerted upon a vesicle of cellular tissue 

 as to stimulate it into a preternatural degree of activity, the 

 result of which is the production of vessels, and the formation 

 of a centre having the power of lengthening. Any cellular 

 matter, which is not of a perishable nature, may be compelled 

 to form buds by a skilful application of heat, moisture, and 

 light. Hence any firm fleshy parts of plants may be employed 

 for propagation, especially fragments of the root, a part which 

 usually possesses an unusually high degree of vitality. A 

 case of the artificial compulsory formation of buds by the 

 scales of a Hyacinth Bulb is mentioned in my Elements of 

 Botany, p. 41, fig. 110. 



There is, indeed, an opinion, which I believe was that of 

 Mr. Knight, that the sap itself can at any time generate 

 buds without any previously formed rudiment; and that buds 

 depend, not upon a specific alteration of the arrangement of 

 the cellular system, called into action by particular circum- 

 stances, but upon a state of the sap favourable to their crea- 

 tion. In proof of this it has been said, that if a bud of the 

 Prunus Pseudo-cerasus, or Chinese Cherry, be inserted upon 

 a cherry stock, it will grow freely, and after a time will emit 

 small roots from just above its union with the stock ; at the 

 time when these little roots are formed, let the shoot be cut 

 back to within a short distance of the stock, and the little 

 roots will then, in consequence of the great impulsion of sap 

 into them, become branches emitting leaves. 



The leaf-buds of the deciduous trees of cold climates are 

 covered by scales, which are also called tegmenta, or some- 

 times pei^ulae ; these afford protection against cold and 

 external accidents, and vary much in texture, thickness, and 

 other characters. Thus, in the Beech, the scales are thin, 



