132 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



may come into play. Thus, when the growth of an organ is mechanically 

 prevented, the same influence of correlation is felt in the other parts as 

 when the organ in question is entirely removed. The formation of callus 

 affords a good instance of how slight differences of pressure may, when 

 combined with correlative influences, affect the productive activity. Thus, 

 according to Tittmann l , a piece of the shoot of Populus forms callus with 

 equal readiness on both cut surfaces in moist air, but only on the upper end 

 when the lower one is imbedded in moist sand. If, however, the upper 

 end is enclosed in a plaster-cast, the callus appears on the end imbedded in 

 sand, so that it always grows where the pressure is least, the difference of 

 pressure acting as the stimulus inducing this particular form of productive 

 activity. Similar powers of discrimination probably play an important part 

 in directing and determining the formation and mode of development of 

 new organs and tissues. Any actual attempt at a new growth, however, 

 is able in the first instance to generate a considerable pressure, and hence 

 a rigid plaster-cast can prevent the outward growth of lateral roots, but 

 not the formation of the primordia 2 . 



Noll 3 observed that new lateral rootlets were produced only on the 

 convex side of a bent root, although primordia formed previously to bending 

 grew out on the concave side as well. The hyphae of Mucor and other 

 fungi behave similarly, whereas bent stems of Phanerogams form roots 

 on all sides, or on none 4 . Although the increased tension on the convex 

 side causes the cell-walls to become thicker there, the above reaction 

 can hardly be directly due to the altered tissue-strains, since it also occurs 

 in Mucor, in which the tension of the convex wall is, it is true, increased, 

 but the external pressure remains unaffected. 



The terms mechanomorphosis 5 , tnechanotropism, and mechanical stimuli may be 

 used in this connexion, and plants may be sensitive to stimuli of tension, pressure, 

 contact, and vibration. No special terms are as yet required for the other forms 

 of sensitivity to mechanical agencies. Verworn terms the results of contact- 

 stimulation thigmotropic and thigmomorphic reactions, whereas Errera uses the 

 terms haptotropism and haptonwrphism*. 



1 Tittmann, Jahrb. f. wiss. Bot., 1895, Bd. XXVII, p. 169. 



a Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 356. 



8 Noll, Landwirth. Jahrb., 1900, Bd. XXIX, p. 422. 



* Vochting, Organbildung, 1878, I, p. 194; 1884, II. p. 45. 



5 Herbst restricted these terms to the results of tension and pressure. 



6 Verworn, Psycho-physiolog. Protistenstudien, 1889, p. 90; Errera, Bot. Ztg., 1884, p. 584, 

 footnote. Sachs (Flora, 1893, p. 9, footnote) suggested the word piesotropism. Loeb (Helio- 

 tropismus d. Thiere, 1889, p. 28) used the word stereotropism to denote the tendency to assume 

 a definite position with regard to the substratum. Somatotropism is used in a similar sense. 



