148 



their pas sage -like course favorfs the explanation of M. 

 Ki^nitz-*-, who recognized in them, at least for Salix, 

 Sarbus and Betula, callus filled passages of the larvae 

 of seme Diptera, Of course, even after other kinds of 

 injury, the cavitie^ produced in the cambium can be filled 

 with similar tissue, 



Histolof^y of the callus 



As has been already emphasized, callus tiesues are char- 

 acterized histologically by the slight differentiation of their 

 cells. In many kinds of cuttings the callus rolls consist ... 

 throughout of the same kind of cells; they are oonstruoted per- 

 fectly homogeneously. The separaate cells are alvmira thin- 

 walled, filled with a clear cytoplasm and an almost always 

 colorless cell sap. In slow growing callus excrescences the 

 tissue is usually small-celled and close, larger interstices 

 are visible only in thS outer cell layers. In those growing 

 rapidly, the cells are Hsually large, loosely, layered, separa- 

 ted especially In the outer layers by large intercellular spaces, 

 In the callus of Oydonia japonioa and others, I found thr.t the 

 tissue connection of 'ten became so broken, that the cells were 

 almost ccHipJetely loosened from one another. If callus tissues 

 are left iji the light, they become green; their ohloroplasts; 

 however, are alv;ays few in number, small and poor in pigfflent; 

 on account of which the callus rolls are always a pale green, 

 at times even more yellow than green, as in Gatalpa, 



No definite laws determining the proportionate sizes be- 

 tween callus cells and those of its mother tissue may be recog- 

 nized^ In Populus, for example, the cambial callus consists 

 of cells, which are appreciablv larger than, those of its mother 

 tissue, while the prodiiots of the pith are smaller than those 

 of normal medullary cells'. 



A difference between the single cells, is caused very often 

 by the fact that some of them, - especially in the inner layer 

 • of the callus,- are changed to tracheids by reticulated thick- 

 ening and lignification of their walls. Their formation may be 

 traced especially .clearly in the callus of poplar cuttings. 

 Figure 67 shows part of a callus of Populus: - the delicate 

 walled, irregularly arranged parenchj^na cells with wide lumina 

 enclose a tracheid v/ith reticulated walls. Although in the 

 (165) "Iiohden wecLge", especially in its lowermost part, very many 



cells assume a tracheal character, tracheids are relatively rare 

 in the upper callus roll. They are irregularly distributed in 

 the undifferentiated tissue. Moreover, the under layers near 

 the place of origin of the roll are usually more abundantly fur- 

 nished v/ith tracheids than the outermost ones. By the uni&n 

 of many tracheids, primitive vascular bundles and a wood-like 

 tissue are produced, which will be spoken of 3ater as wound- 

 wood. The cells of the superficial layer of the callus never 

 become tracheids. 



It is now worth noticing, that not onlyd derivatives of 

 the cambium may become tracheids, but that callus from the 



— . — „ .^...^ -,.-«- --• 



•^ Entsteh, d, Markflecke, Botan, Cbl,, 1883, Bd. XIV, 

 p, 21; there and in DeBary reference to older authors, (Th, 

 Hartig^ Kraus, etc,), 



2 Compare also Sorauer, Handbuoji der Pf^anzenkrankh. , 

 Z aufl, , Bd, 1, p. 38S. 



