ALTERNATION OF GENERATIONS 141 



horizons when certain extinct forms appeared and dis- 

 appeared, but also, in later epochs, to explain anomalies 

 in the distribution of living types. 



ALTERNATION OF GENERATIONS 



The central idea of Hofmeister's work was the recogni- 

 tion of " Alternation of Generations " in the vegetable 

 kingdom. The phrase was introduced by Steenstrup 

 in 1845 to describe cases " when animals produce offspring 

 which do not resemble their parents but produce progeny 

 like the original parent." In botany the term was at 

 first applied only to the succession of vegetative and 

 reproductive shoots. In 1856-8 Pringsheim drew atten- 

 tion to the fact that in the fresh-water Algae, Oedogonium, 

 and Coleochaete, the fertilised ovum did not produce a 

 new plant directly but gave rise to a cluster of cells each 

 of which became a new plant, and compared this group 

 of secondary oosperms to the sporocarp in the lowest 

 Bryophyta. These observations, however, did not arouse 

 the interest they might have done had they been published 

 after 1860, when Hofmeister's results were available. 



Celakowski, in 1868, was the first to appreciate the 

 true bearing of Hofmeister's discoveries. He recognised 

 two distinct stages in the plant's life-cycle, a sexual and 

 an asexual. In the lower plants, Algae and Fungi, he 

 pointed out that the asexual stage was multiple, one 

 asexual generation succeeding another until at length 

 the cycle was closed by a sexual generation, while in 

 archegoniate plants the sexual and asexual generations 

 succeeded each other with perfect regularity. Further, 

 he drew attention to the fact that the two generations 

 in the Thallophyta were similar to each other in all 

 respects, save in the nature of the reproductive organs, 

 whereas the sexual and asexual stages in the Arche- 

 goniatae were profoundly different. Celakowski termed 

 the sexual stage the " protophyte " and the asexual one 



