Allgemeines 387 



Given a stimulable condition the result that will follow on 

 any Stimulus will depend on the nature of the mechanism affected 

 and it may also be modified by the kind of Stimulus applied. 

 In the latter case it perhaps depends on the capacity of the 

 transmitting zone to propagate different kinds of disturbances 

 or decompositions when differently affected. In the alternative 

 case any irritating agent can only set up one kind of change 

 — thus the optic nerve however stimulated only gives rise to 

 the impression of light-, and a Mimosa-leal makes the same 

 response whether the proximate stimulating agency be a lighted 

 match or a pair of forceps. 



Again, recent researches on fertilization have proved that 

 not only is ontogeny initiated by a Stimulus, but that this 

 Stimulus need not necessarily be given by the act of fertilization. 

 Like the optic nerve the structure of the egg is such that it 

 responds to all agencies that can set its chemical machinery a 

 gog, in one way — by segmentation and development. 



Turning to the polarity hypothesis it is difficult to believe 

 that food-material flows upwards to the shoots and settles 

 there as special shoot-forming stuff, or that root-forming 

 stuff descends to the lower regions to make a root-forming 

 mother liquor — how would it on this hypothesis be possible 

 to explain the formation of roots on the shaft of a Martagon 

 Lily or in the case of plants with creeping basal rhizome 

 why do the shoots and roots, though all produced in the same 

 zonal region of the stem, as in Circaea, still retain their mor- 

 phological position in the tangle? 



Though there is strong evidence that nutritive Stimuli do 

 serve as agencies for provoking development and increase 

 in the number of parts in the higher plants, the actual form of 

 the organ to be produced depends on the stimulated mechanism. 

 It is the power of suitably responding to Stimuli that in the 

 long run determines the survival or extinction of a species, but 

 the suitability or the reverse of the response is an accident of 

 the mechanism as a working structure. 



In the case of galls and in the malformations induced in 

 roses by injudicious manuring, we have some idea of the proxi- 

 mal agent at work, but often this is quite obscure, and it is 

 sometimes assumed that we have to deal with a morphological 

 reversion. But it is difficult to think out how a plant can be 

 supposed to replace such a structure as an ovule by a vege- 

 tative bud, or what not, because they are morphologi- 

 cally identical, for it does not replace the ovule by an 

 ancestral type of shoot, but usually by one bearing the 

 character of the sporting plant itself. 



In the tissues that obviously exhibit special irritable pro- 

 perties one finds also strong evidence in favour of the associa- 

 tion of a Stimulus with a material change of the stimulated 

 part. The Stimulus may be mechanically given in the first 



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