144 



PRINCIPLES OF PLANT-TERATOLOGY. 



organ producing the shoots and the shoots themselves, 

 and such stages constitute the only evidence in proof 

 of the identity of any two structures. It seems that 

 the explanation of adventitious shoot-formation is 

 rather to be sought in the nature of the internal 

 mechanism of the organs producing the buds, and 

 that it must really be due to a homology between the 

 protoplasmic constituents, or some of them, of these 

 organs, a germ-plasm being present in most parts of 

 the plant. 



The same argument may be applied to the cases of 

 apogamy and apospory. The protagonists of the 

 theory of "homologous" alternation of generations 

 have seized upon these phenomena as an apt weapon 

 for use in defence of their cause ; if the prothallus, 

 say they, is capable of giving rise directly, by vegeta- 

 tive budding, to a fern-plant, or the fern-plant to a 

 prothallus, this is evidence in favour of the two gene- 

 rations being one and 'homologous. But this appa- 

 rently represents the same indefensible position as 

 that of those who maintain the homology of shoot and 

 leaf from the fact of the latter producing adventitious 

 buds. Why may not germ-plasm exist in the pro- 

 thallus as well as in the foliage- leaf of the fern P On 

 the view above-stated the prothallus is no more 

 necessarily homologous with the adventitious shoot 

 which it bears than is the fern-leaf with the shoot or 

 prothallus which it bears. That which would afford 

 proof of the homological identity of prothallus and 

 fern-plant would be transitional forms (each itself 

 maturely developed) between the two, the one gradu- 

 ally merging into the other ; but no such phenomenon 

 has ever yet been seen. 



It is true that the curious structures induced by 

 Lang in the prothalli of various ferns : cylindrical 

 thallus-lobes bearing ramenta, or sporangia, and con- 

 taining tracheides, and the prothallus-lobes of ordinary 

 flattened form above but leaf-stalk-like in structure 

 below, the " new growths " as he termed them, are 



