WEISMANN'S THEORY OF THE GERMPLASM 49 



difficulties most irreconcilable with Weismami's 

 theory. 



Loeb uses the word ' heteromorphosis ' to denote 

 the ability possessed by organisms, under the 

 stimulus of external forces, to produce organs on 

 parts of the organism where such do not occur 

 normally, or the power to replace lost parts by 

 parts unsimilar to them in form and function- 

 Kegeneration is the reproduction of parts like those 

 lost ; heteromorphosis is the reproduction of parts 

 unlike those lost. 



Heteromorphoses are well known in plant physi- 

 ology. When one cuts a slip from a willow, one 

 may make the cut at the bottom of the slip and 

 the cut at the top in any part of the willow- twig, 

 yet still the lower end of the slip always produces 

 rootlets, which are organs not normal to that part 

 of the twig, while shoots will rise from the upper 

 end. Moreover, either end of the slip may be 

 made the root portion, and it is clear, therefore, 

 that in every small area there are cell-groups 

 present able to bear roots or shoots according to 

 the determining conditions ; and therefore that, in 

 addition to the characters active at any time, there 

 are present the germinal rudiments for shoots and 

 roots, and, indeed, for the whole organism, since 

 the shoots ultimately may bear genital products. 



When the prothallus of a fern has developed 

 normally, it is a flattened leaf-like structure which 

 bears rootlets and male and female genital organs 

 on the lower surface, i.e., on that turned from 

 the light. But the experimenter may reverse this 



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