228 PHYSIOLOGICAL BOTANY. PART II. 



hark. In the earliest stage in which it is discoverable 

 it appears as a thick clammy fluid termed the cambium, 

 which gradually assumes the character of a newly 

 formed cellular tissue intermixed with vessels which are 

 disposed longitudinally through the stem. It should 

 seem that the cellular tissue at least is developed from 

 the old tissue, as may be shown experimentally by 

 grafting a branch containing a wood of one colour on a 

 tree whose wood is of a different colour as a peach on 

 a plum. The new wood retains the distinctive cha- 

 racters of the parts round which it is formed, the graft 

 increasing by pale coloured layers and the stock by layers 

 of a reddish colour, even though these latter have been 

 nourished by the descending sap elaborated in the leaves 

 of the former. Different theories have been proposed in 

 order to account for the manner in which the cellular 

 tissue increases. Some suppose that the young cells 

 are developed within the old ones, which they ulti- 

 mately rupture and replace ; but of this there is no 

 good evidence. Others consider the opaque dots dis- 

 cernible on the surface of some cells to be nascent 

 vesicles, which are afterwards developed on the outside 

 of the old ones ; and this is a more probable hypothesis 

 than the last. According to a third opinion, an old 

 cell becomes separated into compartments by the form- 

 ation of a transverse diaphragm, and each compart- 

 ment afterwards develops into a separate cell. The 

 formation of the fresh vessels is still more ambiguous 

 than that of the cells. One theory considers them ana- 

 logous to descending roots proceeding from the buds 

 placed in the axillae of the leaves, and supposes them 

 to be continuous throughout the whole length of the 

 longest stems. But as vessels are formed, though of 

 small dimensions, in those parts of the stem which are 

 below the place where a ring of bark has been removed, 

 this supposition is untenable. It seems more probable 

 that the vessels have a common origin with the vesicles, 

 or are modifications of them ; and that a long vessel was 

 originally composed of sevetal parts. 



