228 



DISEASES OF TREES 



parenchyma, but these are so surrounded by the elements 

 above mentioned that they are scarcely able to exercise even 

 the limited reproductive capacity which they do possess. This 

 capacity is exhibited in but two forms first, in the production 

 of tyloses or " filling cells " in the vessels of the wod whenever 

 these are injured, and, secondly, in the development of so-called 

 " intermediary " tissue (" cementing tissue ") during the process 

 of engrafting. 1 When the cut surfaces of the scion and stock are 

 bound together in a sufficiently fresh condition, any empty space 

 which may exist between the two portions of wood becomes 

 filled with parenchymatous tissue, which 

 originates in the above-mentioned paren- 

 chymatous cells of the wood itself. 



Wood that is exposed by a wound has 

 the power of producing cortex and wood 

 only if the cortex is removed during the 

 season when the cambium is active, and 

 the cambium layer or the young wood 

 is protected against drought. In such a 

 case the regeneration of the covering 

 layers is effected. The region of the 

 cambium, with its delicate cells and 

 abundant protoplasm, consists, during the 

 period from May to August, of initial 

 cells, mother-cells that have been formed 

 from these by division, and young em- 

 bryonic cellular tissue (young bast and 

 young wood) which is still capable of 



growth. When exposed to the air this region dries up very 

 easily, and only during rainy weather, or when the air is saturated 

 with moisture, does this tissue survive, and, by the transverse 

 division of the elongated elements of the cambium, become 

 converted into a healing tissue consisting of parenchymatous 

 iso-diametric cells. 



Owing to energetic cell-division this gives rise in a few days 



to an investing layer (Fig. 133), which, under the influence of 



light, assumes a green colour. Frequently the cambium that 



covers the surface of a wound withers, with the exception of the 



1 Goppert, Ueber innere Vorgdnge bei dem Veredeln, Cassel, 1874. 



FIG. 133. Surface of a 

 beech-stem from which 

 the cortex has been re- 

 moved, and on which 

 an investing layer has 

 been partially formed. 

 Natural size. 



