235 



impossible that, in the above named "stimulus effect" arid 

 similar ones, some are concerned \7hieh later may be recognized 

 as force effects, v/hen the knovvledge of the mechanics of pto- 

 toplasmas and of the physiology of cells has become more exai5t<, 



That cells may be incited by mechanical pressure and 

 strain to definite processes of growth, division and differen- 

 tiation, is proved by the phenomena of so-called passive 

 growth, by oambial cells brought to cross-division by strong 

 pressure and by the cell division which Kny found in the pith 

 of IiSipatiens and others under the inflaense of pressure, Fur- 

 ther, attention is again called to the activity of hyperplasia 

 of mechanical tisfjue (compare p. 141), 



Thouvenin has shown (compare p, 48) that definite pro- 

 cesses of differentiation can be made Impossible under the in- 

 fluence of raechanicn.1 factors. 



We mentioned above (p. 269) the influence, v/hich, ac- 

 cording to Meigert and others, is exerted upon 'tissue -for- 

 mat ion by the removal of hlnderances to growth, The con- 

 sideration of the action bf mechanical factors, is of the 

 greatest importance here and, in fact, Ribbert"^ has re- 

 cently attempted to explain very different forms of path- 

 ological grov/th - regeneration, inflamation, formation of 

 tiimors - by th© removal of mechanioal hinderanoe to growth . 



So far as the formation of abnormal plant tissues is 

 concerned, not one of their many forms have been suffi- 

 ciently explained as yet by an omission of hlnderances to 

 growth. It may be stated a's very probable that in the pro- 

 duction of callus. v7ound-wood, and others, the removal of 

 bark pressure and similar hlnderances to growth is not 

 without influence on the formation of the wound tissue. 

 However, there is no necessity for the assumption, that the 

 removal of the pressure effect causes the formation of 

 wound tissue. We may find an outgrowth of tissmes of ver;'^ 

 different kinds even v;ithout previous injury ur release 

 from pressure and also of many excrescences (those pro- 

 duced endogenously,- such as galls and intumescences). 



2. Temperature 



All life-processes, and also processes of formation and 

 differentiation, like many physiological or chemical ones, can 

 be carried through only v/ithin certain temperature limits. The 

 formation of tissue on the gro\;'ing shoots and roots will be 

 discontinued if the temperature falls below the requisite min- 

 (277) imum or if it rises §bove the admissible maxiiaitul. But the same 

 temperature limits do not hold good for all processes of forma- 

 tion and differentiation taking place in a growing organ or or- 

 ganism. Certain processes me-v- stil l take place, although others 

 have already become impossible* If unfavorable temperature con- 

 ditions make certain processes of differentiation impossible for 

 developing tissue, "incomplete" abnormal tissues are produced 

 (hjrperplasias) . In many plants, formation of the chlorophyll is 

 rendered impossible by tuoolow a temperature (p. 36). Hans ■ 

 found that the formation of cross-walls in grov;ing cells oi 



X 



Lehrb. d, allg,, Pathologie, 1901. 



