402 ORGANIC GROWTH sec. 



But this is to dispute heredity altogether, it is maintained 

 that the variety of plant -forms is due to external stimuli 

 only, which act upon plants during their development. 

 Whither does such a view lead? To the denial of the very 

 nature of embryonic development, which incontestably de- 

 pends principally on heredity. It leads to the assumption 

 that all germs are equivalent, and that the fact that from a 

 cherry-stone a cherry-tree, from a pea a pea-plant arises, is 

 due only to the difference of external stimuli. 



In a work ^ in which he defends himself against Sachs's 

 attacks, Yochting uses language which even more distinctly 

 than the previous quotations shov/s the agreement between 

 his views and mine, an agreement which I value the more 

 because it was only after the preceding pages had been 

 written that I became accurately acquainted with the con- 

 tents of the work referred to. Amongst other things he 

 says that every organ, of whatever kind it may be, contains 

 internal determining conditions, and these in the cases re- 

 corded by him primarily determined at what points roots 

 should arise in a fragment of willow. Such internal condi- 

 tions " primarily determine the position of new formations. 

 The effects of the action of the external stimuli are thus 

 modified by internal factors." 



These " internal conditions " are the same as those which 

 I have called constitutional causes.^ 



Like Sachs, Pfltiger also denies heredity ^ — and inasmuch 



•^ TJeher Organhildimg im Pflanzenreich, ii. p. 192, Bonn, 1884. 



^ Sachs lias since (J. Sachs, Stoff und Form der Pflanzenorgane, ii., Arh. d. 

 botan. Inst, zu Wiirzburg, Bd. ii. p. 689), not only assumed that there are such in- 

 ternal causes, but even declared that he never denied their existence. It may be 

 remarked that Sachs {NatunoissenschaftUche Rundschau 1886, No. 5) also main- 

 tained that he had put forward the theory of the continuity of the germ-plasm 

 before Weismann (Sachs, Pjianzenphysiologie 1882, ch. xliii. ) But as Weismann 

 replied {Zur Annahme einer Continuitdt des Keimplasmas. Ber. d. naturf. Ges. zu 

 Freiburg i. B. 1886), Sachs only referred to the fact that from the germ-sub- 

 stance of one generation that of the following is derived. 



^ Pfliiger, Ueber den Einfluss der Schioerkraft auf die Theilung der Zellen 



