IRRITABILITY 187 



bly are far from permanent at the utmost. While the effect 

 of one influence still persists, other influences are operating 

 on the organism. The status of the organism at any one 

 time represents the results of all the influences to which it 

 has been subjected up to that time ; its form, size, position, 

 etc., are its response to all these influences. The condition 

 of the organism and the influences bearing upon it, together 

 determine how it will grow. The study of irritability at 

 any one time, therefore, necessarily includes the results of 

 irritability at other times earlier in the life of the in- 

 dividual. 



We may begin our study with those mechanical influences 

 to which plants are or may be subjected. When a growing 

 part is enclosed within a bandage of plaster of Paris, two 

 results follow, one of which has already been mentioned 

 ( p. 174 ) . * The other we may consider now. The part not 

 only lacks room to grow, but the plaster ligature relieves 

 the enclosed part, as it does a broken arm or leg, of me- 

 chanical strain of nearly every sort. The tissues primarily 

 contributing to the mechanical strength of the growing 

 organ attain within the ligature neither such size nor such 

 strength as ordinarily, f From this we may conclude that 

 the mechanical strength of a part depends upon the strain 

 to which it is subjected. This conclusion, reached from ex- 

 periments in which the mechanical strain was reduced as 

 much as possible, is enforced by experiments of the opposite 

 sort, in which. the mechanical strain was increased. t The 

 strain consisted in traction, effected by means of weights 



*Newcomb, F. C. The influence of mechanical resistance on the de- 

 velopment and life-period of cells. Botan. Gazette, vol. 19, 1894. Reg- 

 ulatory formation of mechanical tissue. Ibid. vol. 20, 1895. 



f Pfeffer, W. Druck und Arbeitsleistung. Abh. d. K. Sachs. GeseUsch. f. 

 Wissensch., Bd. XX., 1893. Also Pflanzenphysiologie. 2te Aufl., II., pp. 

 144-7, 1901. 



t Hegler, B. Einfluss des mechanischen Zugs auf das Wachsthum der 

 Pflanze. Cohii's Beitrage zur Biologie der Pflanzen, Bd. VI., 1893. Older 

 literature here cited. See also Pfeffer. W. Besprechung Hegler's Unter- 

 suchungen. Berichte d. K. Sachs. GeseUsch. f. Wissensch., Sitzung vom 7ten 

 Dec, 1891. 47soPflanzenphysiologie.II., §36. Derschau, M. von. Einfluss 

 von Kontakt und Zug auf rankende Blattstiele. Inaug.-Diss., Leipzig, 1893. 



