.821 



and kills them. The dead tissue can be recognized only in small fragments 

 along the wire. 



We have already explained above how these different tissue forms are 

 produced by the sap pressure, at first greatly increased and then nearly 

 removed by the splitting of the bark around the wire. The almost complete 

 breaking up of the split bark makes possible the appearance of wood paren- 

 chyma from the cambial zone ; later, if sap pressure begins because of the 

 uniting of the wound edges over the wire, thus enclosing it, true wood cells 

 and vessels again appear but the arrangement of these elements for some 

 time is horizontal, or spiral, diagonally ascending, caused by the strong 

 pressure of the wire at the time when the cambial zone still lay back of it. 



The extreme twisting of the wood fibres, which can be confirmed also, 

 to a slighter extent normally, in a great many trees, and manifests itself in 

 different degrees in individuals of the same species, is physiologically inter- 

 esting. The twisted groivth is more noticeable in dry places. The greater 

 twisting of the wood fibres, probably caused by the bark of specimens grown 

 in dry places which becomes inelastic sooner, is less easily split and, there- 

 fore, exercises a higher pressure. 



The practical purpose of constriction is the same as that of girdling but 

 without tlie danger entailed by a complete removal of considerable parts of 

 the bark. 



Branch Cuttings. 



The term cutting is applied to any part cut from the parent plant, which 

 by its reserve food materials incites various cell groups, chiefly those near 

 the cut surface, to renewed vegetative increase so that a cicatrization tissue 

 is formed. The separated part by forming new roots develops into an inde- 

 pendent plant. A work by Simon^ throws light on the anatomical conditions 

 and the dependence of tissue differention on external factors, which appear 

 during the pressure and can not longer be taken into consideration. 



It may be asserted that an asexual propagation of this kind may be 

 found in all classes of the vegetable kingdom and may take place from very 

 different organs. We recall here the continued growth of torn off mycelial 

 threads, of cut sclerotia, of isolated fruiting stems of the frondiferous 

 mosses and of leaf and blossom parts of phanerogams. Beside the fre- 

 quently occurring root cuttings, cases have also been known of the formation 

 of roots from fruits. 



We are concerned here for the present with cuttings from branches, 

 the cut surfaces of which react to the wound stimulus by the formation of 

 callus. In connection with this, we will then discuss propagation by root 

 cuttings, the cicatrization of which also begins with the formation of callus. 

 The transformation of the callus into an actual overgrowth edge by the 

 formation of a peripheral cork zone bears very great resemblance to the 

 formation of the overgrowth edges on girdled, or transversely cut, woody 



1 Simon, S., Experimentelle Untersuchung-en uber die Differenzierungsvorgange 

 im Callusgewebe von Holzgewachsen. Leipzig 1908, Gebr. Borntrager. 



