Extern a l form of the callus 142 



—— il M lll |l| H I III ■!■ ■»!■ —I I I 1 llh»<MMI— M II I m il ll I IPIII 



The prominent activity of the canbiura in callus forma- 

 tion, tletormines the fact that the oaXius, at least in its 

 first stages, usually appears in the form of e; ring on the 

 cross section of the shoot. In cuttings of me,ny plants, the 

 production of callus is relatively scanty, so that in them it 

 (156) never passes beyond the stage of a flat, ring-llk6 roll. In 

 others, as populus, Rosa, Cntalpr and many others, the callus 

 continues to g rov/ for a longer time, especially if the cuttings 

 are kept in moist air. The fact that callus tissue never has 

 a specific form is also important. The luxuriant new forma- 

 tions, producer;, for example, on cuttings of populus T^yramid - 

 alis (compare fig. 60) assu^Jie very different forms, even when 

 allowed to develop fpeely, and are covered v/ith large and 

 smr.ll protuberenoes, irregularly distributed. Besides this, 

 from the very beginning, the production of callus in many eases 

 does not take place equally strongly on all parts of the cut 

 surface,- often indeed it mayfte entirely lacking in places. 

 The space conditions of the callus often determine its form; 

 the callus masses fill out all splits in the wood of the cut- 

 ting, eto^ Figure 60B shavs a callus which was developed un- 

 der a glass plate* Since development in a longitudinal direc- 

 tion was, thus prevented, it spread out sidewise without being 

 distinguishable In any way from callus tissue which develops 

 undisturbed. The same is tnie of the cal3.us tis£5ue-s of herba- 

 ceous plants. Figure 61- shows the callus from a stalk of 

 Xamium orvala . the plant which among herbaceous plants produces 

 the largest cellus-roHls of which I know. The ir»egular, wax- 

 yellow callus masses, as large as one centemeter ^^'hich are pro- 

 duced on cuttings of Catalpa s^/ringae folia dre very striking 

 and resemble coralloid "branches cones, ^ 



In 'rare oases the callus appears in the form of algae-like 

 threads, as Stoll for example has observed in TradeScantla, At 

 this point, I would like to mention the hair-like structiXres 

 found by Sorauer ifl the core of "woolly streaked" apples, 

 (loc. olt, p« 296), which might possibly b)e attributed to in- 

 jury. (Compare fig, 68). Massart also (loc. clt. p. 56) ob- 

 sprved the same structures. I will return later to these. 



The period of development of the callus varies with th© 

 controlling external conditions and the nutritive condition 

 of the object under ihteetigation. 



Origin of the callus 



Callus tissue can be produced on all organs, on roots, 

 axes, and leaves. Yet all parts of all plants are not capa- 

 ble of forming it, 



Further, all living elements of exposed tissue can Je 

 incited to growth and division through injury so far as their 

 membranes have no* become lignified, and therefore incapable 

 of surface-growth. An exception to this is mgde by wood par- 

 enchyma cells which, as seen above, are incited only to growth 

 by wound stimuli, overlooking some very rare division, usu- 

 ally in wounded organs and isolated pieces of organs of any 

 definite plant species, only single tissue forms are caused 

 to form callus. 



The productive pov^'er of different kinds of tissue varies 

 greqtly. In those rare cases in which the epidermis sots at 

 all it produces only very small amounts of tissue. The ohiej 



