328 METAMORPHOSIS 



In addition to the simple healing of a wound by means of cork we meet 

 in other cases with a more complex method, viz. the formation of callus. The 

 cells which remain uninjured near the edge of the wound begin to grow actively 

 and curve over it ; cell division begins and finally develops there a thin-walled 

 large-celled tissue of irregular form to which the name of callus is given. All 

 cells containing protoplasm and nuclei, even epidermal cells, seem capable of 

 forming callus, but, naturally, young cells, and especially cambium cells, are 

 more vigorous than full-grown cells. [This capacity for forming callus disap- 

 pears in some cases early, in other cases late ; very rapidly, for example, in 

 the cells of the root cortex (MASSART, 1898 ; SIMON, 1904).] Small wounds, 

 such as pricks in leaves, may be completely occluded by callus alone, but in 

 larger wounds a development of the cortical cells takes place as well, accom- 

 panied by suberization of the outermost callus cells. In this way a replacement 

 of the epidermal cells is attained, always the first object aimed at. This is, 

 however, by no means the only purpose of the formation of callus. Without 



attempting to offer a complete exposition 

 of the subject it will be sufficient if we 

 cite only a few examples here. Let us 

 glance first at callus formation on cuttings. 

 As is well known, gardeners propagate 

 many plants by very simple means, viz. 

 by cutting off twigs, placing them in wet 

 sand, and keeping them warm. If the 

 plant has the power of forming new roots 

 under these conditions then propagation 

 by cuttings is possible, otherwise not. 



Fig. 08. Callus formation. / longitudinal sec- -/ /. ? , e . , ... 



tion through the base of a three days' old cutting of Bef Ore TOOtS are f Ormed f rOHl the Cutting 



^S^^^^T^^^& there is formed at its lower cut end, 

 base of a one-year-oid cutting; m, medulla ; x^ imbedded m the sand, a callus, formed 

 Th1o y enfe from the cambium, phloem, paren- 



After STOLL (1874, Bot. ztg. 32, PI. 12). chyma, and pith, as shown in Fig. 



98, /. It will be seen from the figure that 



the cambiogenic callus cells overlap each other and thus the callus tissue comes 

 to form a hemispherical cushion on the cut surface of the slip (Fig. 98, //). Simi- 

 larly other callus masses develop on the cut surface, and by their fusion form 

 in the long run a single hemispherical mass of tissue. When the wound is 

 quite covered, corky layers appear in the callus which enclose the lower ends of 

 the vascular bundles, and separate them from the new tissue. On the outer 

 surface of the callus also a corky layer has already appeared. Later on, at 

 a certain distance from this layer, a cambium develops which unites with that 

 of the cutting and proceeds to form secondary wood inwardly and secondary 

 bast outwardly. In a one-year-old cutting, therefore, we find the secondary 

 tissue arranged at the base from within outwards as in the stem and in direct 

 continuity with it. We shall direct attention later on to the formation of 

 roots from the base of the cutting ; these are not shown in the figure. 



The callus in this case is able to form secondary tissues, but in other 

 cases we find the callus capable of producing primary tissues. If, for 

 example, we cut off the extreme apex of the growing point of a dicotyle- 

 donous root, the callus which arises is speedily able to produce a new growing 

 point, which performs its functions quite normally and forms a new rootcap 

 [SiMON, 1904]. This property is, however, almost confined to the root ; it is 

 seldom possessed by the stem (PETERS, 1897). 



As a rule, when an organ is removed its renewal does not take place from 

 the callus exactly at the same place, but in the neighbourhood ; this new organ 

 may be already present previous to the infliction of the wound as a more or less 



