CONSERVATIVE ORGANS. [LECT. VII. 



trees the longitudinal threads are seen lying 

 nearly parallel to one another, without any meshes 

 or intervening cellular matter. Like the other 

 parts of the bark, the liber is annually reproduced. 

 The old layer loses its vitality, and is pushed out- 

 wards by the new ; the accumulation thus formed 

 constituting what botanical writers have called 

 the cortical layers, which Malpighi supposed de- 

 rived their origin from the older bark. 



The vitality of the stem of dicotyledonous 

 plants is more conspicuous in the liber than in 

 any other part. If the bark be wounded, or a 

 portion of it be removed, layers gradually extend 

 themselves from the liber on each side of the 

 wound until it is closed up ; but, as this is not ef- 

 fected in one year when the wound is extensive, 

 and as the new layers are thrown out by the liber 

 only which is annually renewed, the cicatrix, if 

 the healed portion can be so named, always re- 

 sembles a hollow cone, the base of which is the 

 exterior of the trunk. The union of a graft, or of 

 a bud taken from one tree and implanted on ano- 

 ther, succeeds only when the liber of the bud, 

 or the graft and that of the stock is placed in 

 immediate contact ; the union in these instances 

 closely resembling that which occurs when two 

 raw surfaces of a living animal body, or of two 

 distinct animals, are retained for some time in con- 

 tact. Grew, Malpighi, Du Hamel, and others 



