336 



METAMORPHOSIS 



in their neighbourhood (JosT, 1893). We are here deahng with a quite special 

 kind of effect — a stimulus action — which comes from the growing organ and 

 propagates itself backwards. [Exceptions to this view are taken by Monte- 

 martini (1904).] We are unable to say, however, whether any definite chemical 

 substance exudes from the leaf which acts as a stimulus. The similarity 

 between the influence of the growing leaf and that due to the gall insect is worthy 

 of note, and this likeness would tend to suggest that we are dealing in this case 



also with a chemical stimulus. Doubt- 

 less stimuli such as we meet with in 

 Phaseolus maybe frequently met with, 

 and especially in the relationships of 

 secondary growth of the main axis to 

 the development of lateral organs, 

 relationships which are easily under- 

 stood from a biological point of view, 

 though from a causal aspect they are 

 not intelligible. 



An attempt has also been made 

 to explain the phenomena of regenera- 

 tion causally. But a causal explana- 

 tion is naturally not provided by 

 putting forward the appropriateness 

 of the regeneration or by describing it 

 as a ' tendency to completion of the 

 body '. On the other hand, we more 

 nearly arrive at a causal relationship 

 after determining (compare p. 331) 

 that the inhibition of the function of 

 an organ may lead to regeneration as 

 readily as its complete removal. There 

 are two recent researches, one by 

 GoEBEL (1903), the other by Klebs 

 (1903), which may be noted here as 

 throwing light on the subject. We 

 will select only one important item 

 from Klebs' s paper. This author 

 starts from the position — which must 

 be accepted by all investigators — that 

 every organ laid down in the plant 

 must develop if all the conditions of 

 its growth are fulfilled. He recognizes 

 as one essential condition of the growth 

 of the root a certain amount of water 

 in the plant. Since a branch of a willow 

 cut off from the main stem is able by 

 developing roots to grow into an 

 independent plant, that, according to 

 Klebs, can be accounted for only by 

 an extra supply of water to the root primordia. Again, roots may be induced 

 to shoot out at any place from willow twigs while they are still united with 

 the plant if we permit the entry of sufficient water to their primordia by 

 removal of the cork. Thus Klebs was successful in getting roots to develop 

 (Fig. 103) at the apex of twigs of willow, although up till then an upsetting of 

 internal polarity had not been possible. 



It is possible that the correlative inhibition of many buds by the chief 



B 



Fig. 103. Two cuttings of Saiix pen/aiidra, the 

 upper ends freed from cork, the lower ends immersed in 

 water, cultivated in a very moist atmosphere at a tem- 

 perature of about 25°. A, in the normal position, with 

 callus above and roots below ; B, with i oots developed 

 from the (inverted) apical region. 



