The Life of the Plant. i 181 
tive cambium is excited into activity, masses of succulent cells 
are formed, swell up to a certain extent between the inner- 
most layer of bark and the wood, and spread laterally over 
the cut ends of both xylem and phloem, these latter taking 
no part in the formation of new tissue (Fig. 352). ‘Thus is 
formed a spongy, cartilaginous, succulent cushion of tissue, 
known to gardeners as a callus. At the same time the 
outer wounded parts of the bark, and sometimes also the 
pith, are protected from further injury by a newly formed 
cork-tissue. In cuttings (Fig. 352), z.e. cut branches planted 
in the earth, the roots are developed either 
through the tissue of the callus or near it, 
but always directly from the cambium. 
In grafting, layering, and budding, the 
cambium plays the same part as in the 
cutting, giving rise to the recuperative 
mass of healing tissue, or callus. This 
forces itself into the injured space between 
the bark and the wood (Fig. 351), fills it 
Fic. 351. — Diagram- _ Fic. 352.—Diagrammatic representation of a longitu- 
matic representation dinal section through the lower end of a cutting; cZ 
of a longitudinal sec- the callus ; cv cambium ; A wood; & cork; pith; 
tion through a graft; ry bark ; w roots. 
cl the callus; ~ bark ; 
we pith. 
up in all directions, covers up the dead parts of the tissue, 
and causes those parts that are still active to coalesce with 
one another. The various modifications of this process are 
