151 



Similar differences exist, in regard to the quality of 

 the canua tissue produced. PopulUB heads the list of the 

 (168) many woody plants with cuttings of which I have worlced. In 



moist air masses of tissue often mor« than one centimeter high 

 and wide grow out on the cut surface. Cuttings of many other 

 woody plants produce ohly a low callus ring (Ulntts, Salix, 

 Ostrya, Quercus, etc). Furtlier^, in Ulmus and Populus, the 

 stimulus proceeding from the place of injury also makes itself 

 felt at a considerable distance from the cut surface. Through 

 abnormal cambial activity, the bark is distended until it may 

 at times even rupture. In most other woody plants abundant 

 cell division takes place only in the immediate vicinity of the 

 place of injury/ 



We will have to trace these and similar differences in the 

 foraation of callus to specific peculiarities of the plant 

 species concerned, v/hen a closer analysis of the causes may 

 not be possible. 



As may be surmised, the nutifeitive condition of the object 

 under investigation will influence the formation of callus. If 

 we compare the organs of one plant which are rich in food 

 stuff with those of the same species poor in food stuff, we 

 find that the former develop wound tissue more abundantly than 

 the latter, 3?he difference between the action of the cotyle- 

 dons of many Leguminoseae is most striking. (Vicia, Phaseolus, 

 etc.). Rich in proteins and starch, they proliferate the tis- 

 sue on the cut surface to an extra-ordinary degree, but the 

 thin leaf blades of the same plants develop only weak callus. 

 We find the same difference betv/een cotyledons and foliage 

 leaves of the Cucurbitaceae , fLuffa, Cucumis, etc.) and others. 

 The fact that a more luxuriant callus is formed in ini^^^red 

 leaves (cotyledons) in the immediate vicinity of the stronger 

 ribs than between them, may likewise be traced to unequal nu- 

 trition. Ho investigation has yet been made as to whether sup- 

 pljring food stuffs artificially from without, treatment with 

 sugar solution and the like, might promote the callTis formation 

 of organs, poor in food stuffs. 



Concerning the "external" conditions which may be raoitiiied 

 as one wishes Jn the experiment and their effect studied, the 

 following should be emphasized. 



The influence of necessary moisture on the injured tissue 

 is a condition preliminary to every callus formation, whether 

 it be in a liquid or gaseous form; callus may be formed under 

 water as well as in moist air, but not in dry air. Many plants 

 indeed form callus, only in moist air, and the callus formation 

 in all plants is much more abundant in moist air than under 

 water - supposedly because the absorfjtion of oxj'gen and trans- 

 piration are hindered under the latter conditions. Gravity 

 and light are not decisive in the formation, or, rather, the 

 non-formation of the callus. Indeed it developes often more 

 abundantly in cultures in the dark than in the light (for in- 

 stance, in Populus); perhaps the humidity of the air, increased 

 in the dark, is the cause of this. Of cotirse, there is no for- 

 mation of chlorophyll in the dark. 



If a cutting be left with both ends in moist air, it de- 

 velops abundant callus on both cut surfa6es. If one end is in 

 &Ty air, which, as previously stater', prevents all callus for- 

 mations, the callus tissue can develop only on the other end, 

 tiea) where conditions are favorable. If one end is in water or aund, 

 the ether in moist air, the latter is then preferred and de- 



