THK TREATMENT OF THE SHOOT 



151 



layer makes its appeai-ance ; this is a layei* of constantly 

 dividing cells which runs parallel to the old wood {It), and this 

 layer behaves like a true cambium layer, forming new elements 

 of wood towards h, and new bast cells towards m, I, k. In 

 this way the surface of the stem which had been laid bare 

 covers itself afresh with a new bast. 



Fig. 24 represents another method of callus formation, 

 which we shall refer to again in speaking of grafting. Here 

 the callus is formed entirely from the cells of the pith. The 

 figure represents one side of the innermost angle of a wedge- 

 shaped cut which has been made into the wild stock for the 

 purpose of inserting 



the scion. The cut ^f// 



has reached down to 

 the pith (m), and has 

 cut the woody cylinder 

 (h) along the line scJi. 

 The opposite side of 

 the wedge-shaped in- 

 cision has been left 

 out of the figure. In 

 consequence of this 

 cut a growth of rather 

 rare occurrence can be 

 noticed — namely, the 

 growing out of the cells 

 mJc at the juncture of 

 wood and pith. They 



have not only grown out, but have divided to form a con- 

 siderable amount of callus, reaching from c to c. This 

 callus mass grows into the crack which exists between the 

 stock and the scion, and aids the growing together of 

 the two, which is, however, chiefly caused by a similar but 

 more abundant growth of callus from the cambium layer. 

 Crystals of oxalate of lime (0) are seen at intervals in the mass 

 of callus. 



In Fig. 2 5 we see the method of healing and the formation 

 of callus from the bast and cortex. The cortex and bast 

 were cut through in the direction of the arrow, the cut 



Fig. 24.— Formation of Callus in the Pith of a 

 One-Year-Old Twig op TUia europcea. 



