166 THE CAUSE OF SPECIFIC SHAPE 



of a bud is mechanically prevented as when it is removed, if we exclude 

 the effects due in the latter case to the wound-reaction. 



Various agencies are employed in correlation, and among these the 

 consumption and distribution of food-materials are included. It is, however, 

 incorrect to suppose that the growth of a dormant bud is prevented by an 

 insufficiency of food ] , for such buds are often stored with food-materials, 

 and begin to shoot when isolated, while, further, a mechanical restriction of 

 growth may induce a retardation of growth in related organs although 

 these have now a greater supply of food at their disposal. Starvation 

 of course ultimately causes a cessation of growth in growing organs, 

 although only when it is far severer than it is ever likely to be in any 

 organ of a well-nourished plant. 



Similarly, correlative influences, and not a deficiency of food, determine 

 the early death and emptying of those cells which are converted into 

 tracheae. The same applies to resting buds both in the growing plant and 

 in the seeds, and these may die without ever being awakened. Owing to the 

 non-performance of their functions many petioles die after the removal of 

 the lamina 2 . Further, certain bacteria suppress others, not by devouring all 

 the available food, but by producing special metabolic products which act 

 as poisons to other species. 



Functional stimuli. These are concerned in all correlative actions, and not 

 only direct the growth and transformations of cells, organs, and tissues, but may also 

 induce the formation of new organs, such as chloroplastids, which perform a function 

 not hitherto exercised by the young cell or tissue. In this case the function can 

 hardly act as a stimulus to the formation of the organs for its performance, but such 

 a general function as translocation, which is possible in all undifferentiated tissues, 

 may co-operate in inducing the differentiation of vascular bundles which then 

 act as special channels for translocation 3 . The fact that a realized functional 

 activity may act as a stimulus to further development is a fundamental phenomenon 

 of life, and it is hardly surprising that the effects of such functional activity may 

 spread to other parts and produce in them very varied results. 



1 Vochting, Organbildung, 1878, 1, p. 232 ; 1884, II, p. 113 ; Th. Hartig, Bot. Ztg., 1862, p. 82 ; 

 R. Hartig, Lehrbuch, 1891, p. 235 ; Massart, La cicatrisation chez les ve'getaux, 1898, p. 61. 

 The non-development of the terminal buds of Fagus, U/mus, &c., is assuredly not directly due to the 

 withdrawal of water from them, as Wiesner supposes (Bot. Ztg., 1889, p. i). The relative water- 

 supply may, however, exercise a stimulatory effect. Cf. also Goebel, Organography, 1900, I, p. 207. 



2 Jost (Bot. Ztg., 1891, p. 530; 1893, p. 131) and also Busch (Ber. d. Bot. Ges., 1889, General- 

 vers., p. 29) state that no leaf-trace is formed in the petiole of Phaseoltts when continuity is broken 

 at a sufficiently early stage. Apparently the translocation between graft and stock determines the 

 differentiation of vascular bundles in the connecting callus-tissue, and so places the vascular systems 

 in direct continuity. The differentiation of the embryo is, however, entirely due to internal causes. For 

 instances of the influence of use upon development cf. Sects. 34, 36, 46. 



3 Hertwig (Zelle u. Gewebe, 1898, II, pp. 100, 172), Roux (Gesammelte Abhandl., 1895, I, 

 P- 33 1 )) ar >d Driesch (Analyt. Theorie d. organischen Entwickelung, 1894, p. 62) differ in detail as 

 regards questions of origin. 



