154 



The phenomena here described are most closelv connected 

 vath the action of unlike external conditions already discussed. 

 The "polfarity" exprevssed in the callus foria?-tion of Taraxacum 

 (171) roots does not rest entirely upon the fact that the apical cut 

 surface uas perrmnently, or at least at the beginning of the 

 experiment, incapable of forming callus. If root cuttings of 

 Taraxacum are inverted and placed in rmter, in such a way 

 that only the apical poles extend into the air, callus is form- 

 ed on the latter uhile the tiasal surface remains free froi^ it ' 

 just as when inverted specimens are put in sand, or in plaster, 

 etc. The polarity then seems to be "reversed". Poplar cuttings' 

 behave similarly. If they are put in water in a normal position, 

 the basal pole is not only unible to d evelop a more luxuriant 

 callus, than the rpioal one but its callus, if developed at all, 

 is leSvS than that of the other pole. If both poles are placed 

 under the same external conditions land one pole nevertheless 

 seems to be preferred, or if only one of the t\vo develops ogl- 

 • lus, this proved only that the . internal conditions . likevrise 

 affecting the forn^-tion of callus, are unlike at both poles and 

 that, as ali?ays, that pole shavs the most abundant tissue pro- 

 duction, in V7hioh exist conditions more favorable for tissue 

 formation. It is difficult to say, vhat these internal condi-f- 

 tions are. I suppose that here, as is so often the ease, in- 

 equalities in nutritive conditions decide the mp-tter, and I re- 

 turn to the old assumption of a decreased flov^ of the sap, in 

 order to explain the jr eferenoe of the basal pole in pieces of 

 the stem, leaves and petioles. Taking this kind .of sap current 

 for granted, an accix^lation of food substances on the basal 

 pole ipay well be conceived. Analogously, a stream directed 

 toward the upper end, must be assumed in roots, in order to ex- 

 plain the superior nutrition of the upper end of the cutting. 



The processes of tissue differentiation depend also 

 on definite external conditions. We can study the action 

 of arresting factors in abnormal tissues, as in normal 

 ones. It is a matterm^f course that the cells of the cal- 

 lus roll remain free from chlorophyll, when shut away from 

 light. In Populus the wound tissue remains snow white., in 

 others it assumes a yellowish tone. The fact that I could 

 find no tracheids in the callus of cuttings of Amygdalus, 

 Cfefrylus and Porsythia is undoubtedly connected with the con- 

 ditions under which the callus was produced (too slight 

 transpiration?), L more exact testing of the question 

 was not undertaken. 



It should be noted in conclusion, that callus tissue can 

 be produced in the plant body even in the "normal" process of 

 tissue form£»,tion. Such a case occurs if the "mechanical ring" 

 in shoots of many nlants i« finally broken by continued secon* 

 dary growth in thickness. The "physiological" wound thus pro- 

 duced is soon Glos-ed by an in-growing parenchyma, comparable 

 to callus. 



Formation of organs in the callus 



In verv many woody plants, organs of different kinds are 

 stimulated. by injury to the formation of adventitious vegeta^ 

 tive points - adventitious shoots and adventitious roots. These, 

 (172) vegetative points are thereby produced directly from cells of 

 the permanent tissue of the wounded plant organs, or a callua 

 is first formed and from this the gegetative points. Adventi- 

 tious structures of the first kind ©x^st in many ferns m the 

 leaves of many monocotyledons etc. Adventitious structures or 

 the second kind in leaf -cut tings of Peperomia, in the shoot- 



