174 



THE PHYSIOLOGY OF PLANTS 



Care must therefore be taken that the medium in which the 



cutting is placed is very thoroughly aerated. 



When the closing of the wound begins, the cells of the 



cambium, of the young layers of the wood and of the bast 



begin to absorb more water and to bulge out over the cut 



surface. When the delicate extended portion of the growing 



cell has attained a certain length, a transverse wall is formed 



behind the apex, and the latter grows on farther. As the 



cells, which bulge out in this manner and divide, are very 



closely set, long rows of cells will soon be formed which go 



on growing at one end, and 



being firmly packed together, 



form a delicate white tissue 



(Fig, 26). As long as this soft 



tissue continues to grow at 



one end, and thus increases its 



bulk, it is termed callus. 



But, after a while, the 



plastic substances which are 



being passed down from the 



cutting to the callus find the 



path through all the callus 



cells to the extremity too long 



to traverse. The diffusion of 

 Fig. 26.— Very Yodng Callus, the Cells 



OF WHICH CONTINUE TO GROW AT THEIR tho lood matter docs not take 



r\S/crS(.)™^"'''''^"pl^'^« sufficiently actively to 

 6. bast ; /i. wood cells ; m. cambium. the margin of the Call US, and 

 the growth ceases at that 

 region. Instead of that within the callus, an arched strip of 

 tissue makes its appearance, and its cells continue to increase 

 in number. This strip of meristem becomes confluent with 

 the cambium of the cutting, and represents the continuation 

 of the cambium within the layer of callus. Within the latter 

 it now commences to form bast cells on its outside, and on 

 its inside new wood elements, and this forms the actual 

 covering layer. The covering layers at the lower end of the 

 cutting do not difier in any essential manner from those 

 formed in the healing of a pruned branch, except that no green 

 colouring matter is formed, owing to the absence of sunlight. 



