LETTER XII, 



127 



' The accompanying figure (Fig. 15) will aid us in the 

 understanding of this. It is the vertical section of a 

 tree three years old, as it exists in autumn after the fall 

 of the leaves and flowers and fruit. At the summit of 

 the terminal shoot, there is a bud {a), and proceeding 

 from the base of this downwards, and lying between 

 the bark (6 h) and the stem and root (c c) of that 

 shoot, is a layer id d, d d, d d) of cells, constituting 

 the cambium layer. This layer may in a certain sense, 

 and that a very correct one — if not, indeed, as I be- 

 lieve, truly and physiologically, — be regarded as an 

 EXTENSION of the BUD, and a constituent part of it. 

 Formed concurrently with the bud, and in common 

 with it, out of the general cellular basis, it may like- 

 wise be regarded as an adaptation of the bud to the 

 special circumstances of tree-plants parasitic plants, 

 and to the special objects of their existence as timher- 

 producing plants. 



10. Until a comparatively recent period, the minute 

 structure of this layer was but imperfectly known, and 

 its real nature so far misunderstood.* It is not what 

 it seems to be — a glutinous or mucilaginous fluid, — 



* " Pour nous, le Cambium est toujours le Jluide nutritif, pro- 

 duit de la seve elaboree, qui s'epanche au printemps et en automne 

 entre le bois et Tecorce." " Le Cambium est de fluide essentielle- 

 ment nourricier du vegetal, comme le sang pour les animaux." " line 

 devient pas tissu cellulaire, ni tissu vaseulaire; mais ces tissus deja 

 existans y puisent les principes au moyen desquels ils se multiplient.'* 

 — Richard, Nouveaux Elemens de Botanique, 6iesin.Ed. (1833), p. 111. 



