833 



generally in Germany but now only for isolated, special cases to rejuvenate 

 older trunks. Cleft grafting consists of pushing a scion, cut wedge shaped 

 on two sides, into a cleft in the stock which has been cut off square. This 

 cleft is produced by splitting or by cutting out a wedge from the wood. 



In considering the processes of healing, i. e. processes of union in the 

 different forms of grafting, we must distinguish first of all whether this has 

 been carried out on soft w^ood, or on branches of mature, strong wood. In 

 the first case more tissues participate in the formation of the "layer of 

 union" than in the latter case in which a mass of tissue is chiefly involved, 

 formed from the cambial zone (at times also from the pith zone). This 

 tissue forces itself into the space between the scion and the stock, or figur- 

 atively speaking must pour in between the two adjacent parts. 



OCULATION OR BUDDING. 



The most interesting processes of union are found in oculation. In the 

 plate here given, a budded rose is pictured. In one-half of this drawing 

 (from I to ^), the tissue structures are shown after six days; in the other 

 half (from 2 to 5) after about four weeks. The section through the place 

 of budding clearly shows the inserted bud at E, the stock at w. In the 

 stock, hh is the old wood of the previous year, sh, the wood of the current 

 year, formed at the time of oculation. R L are the bark strips, raised by the 

 T-cut; in them, h should indicate the phloem fibres, t the dead tissue of the 

 cut edge. 



At the time the bark strips were spread out from one another by the 

 inpushing of the bud (£), the cambium was very active. The raising of 

 the bark takes place here in the sapwood in such a way that the youngest 

 vascular primordia {g) and the cambial layers (c) lying in front of them 

 remained attached to the bark strips. 



Often only the bark is raised. In fact, under some conditions, pieces 

 of the entire cambial region with the youngest bark cells remain attached to 

 the wood. No evidence of any fixed law has been recognized in this con- 

 nection. It seems that the momentarily tenderest part is torn when the 

 bark is raised and that individual homologous tissues can behave differently 

 at the same time in the same varieties ; in fact, that even the bark on the 

 different sides of the trunk has a different loosening quality. Therefore, 

 the processes of healing are unlike in the same species and variety even in 

 the same grafted individual at different heights. 



Even after 12 hours a change in the peripheral cell layers may be recog- 

 nized on the edges of the wound in the bark as well as in the wood ; the 

 walls of these cells have thickened and turned yellow, either on the exposed 

 side alone, or on all sides of the cell ; the cell contents have increased. It 

 cannot be determined whether this has taken place only because of swelling, 

 as in the wall, or by the transference of material from the inner part of the 



