jpl^ Dr. A. Braun on the Vegetable Individi^^h 



considered that the significance of alternation of generation con- 

 sisted in its being an organic nursing of the brood connected 

 with particular generations, for which reason he termed the in- 

 dividuals of these generations " nurses •/' — a mode of viewing 

 the subject, which, with all Steenstrup's pregnant elaboration of 

 his idea, and with all the analogies he pointed out between it 

 and the well-known phsenomena of nursing the brood by parti- 

 ticular individuals among bees, wasps, ants and termites, does 

 not seize the essential point of the phsenomenon of alternation 

 of generations*. R. Leuckartf conceives alternation of gene- 

 ration from a more comprehensive physiological point of view, 

 in connexion with the totality of all the other phsenomena of 

 the formation of different individuals, whether it occurs in a dif- 

 ferent or in the same generation ; regarding all these phseno- 

 mena from the point of view of a division, not merely of the 

 generic task, but of the vital task in general, among certain in- 

 dividuals ; considering it as a polymorphism determined by a 

 division of labour. But even this view must lead to the mor- 

 phological one ; for the division of labour is determined by the 

 organic development, while this itself obtains its peculiar cha- 

 racter from the determinate step of the metamorphosis at which 

 the development ceases; — and this is just what is so unmis- 

 takable in the phsenomena of alternation of generation in plants. 

 Hence as a typical phsenomenon of development, as a metamor- 

 phosis of generation, alternation of generation (as well as the 

 metamorphosis of the individual) presents analogies with the 

 graduated series in the animal and vegetable kingdoms, and 

 the organic scale of the creation, in general ; — a point to which 



* Steenstrup's explanation is most correct in regard to the history of the 

 development of Distoma, whose nurses and grand-nurses are at last utricles 

 entirely filled with the brood, and forming mere receptacles of the brood. 

 Its application is less happy to those cases where the transition from the 

 preparatory generations to the final generation takes place through external 

 shoot- or bud-formation, as in Sertularice, Campanularice, and CoryncBy 

 whose nurses forming the polype-stem can continue to live even after the 

 concluding generations, comparable to the flower in plants, separate or 

 wither off. Hence the vital activity of the preparatory generations is not 

 exhausted in the production of the brood. Steenstrup's view, accordingly, 

 would only be correct if non-sexual brood-production (by internal or ex- 

 ternal shoot-formation or by division) and alternation of generation were 

 correlative conditions of each other. But this is not the case, as repro- 

 duction by shoots takes place without any alternation of generation in a 

 great number of animals {Ascidia, Bryozoa, Madrepora), and by division 

 as well (Astnea, Annulata, Infusoria). These cases are comparable to the 

 occurrence of unessential branches in plants -, while alternation of generation 

 represents the succession of essential shoots. 



t Ueber d. Polymorphismus d. Indiv. od. d. Ersch. der Arbeitstheilung 

 in d. Natur. Ein Beitrag z. Lehre v. Generationsw. (1851). .^j^j^ £xy..,. 



