Vegetable Morphology. 47 



Thus, in a recent retrospect Prof. Marshall Ward 

 writes as follows : 



" Bower and Campbell have laid bare, by their indefatigable 

 labours, the histological details of the Mosses and Vascular 

 Cryptogams, and carried the questions of alternations of gener- 

 ations and the evolution of these plants so far, that it would 

 almost seem little remains to be done with Hofmeister's brilliant 

 conception but to ask whither it is leading us ; the genetic re- 

 lationships have become so clear, even to the details, that the 

 recent discovery by Ikeno and Hirase of spermatozoids in the 

 pollen-tubes of Cycas and Gingko, almost loses its power of 

 surprising us, because the facts fit in so well with what was 

 already taught us by these and other workers ". 



The idea of alternation of generations came to botany 

 from zoology through the influence of Steenstrup's 

 famous essay. It was established by Hof- Alternation 

 meister (1851) in regard to mosses, ferns, ofGener- 

 conifers, and the like, where he showed the atlons - 

 regular alternation of a sexual and a spore-bearing 

 generation. The sexless "fern-plant" produces spores; 

 these develop into minute sexual prothalli, from the 

 fertilized ova of which the "fern-plants" arise. The 

 sexual "moss-plant" produces ova and spermatozoa; 

 from the fertilized ovum there springs a "moss-capsule", 

 which remains attached to the "moss-plant", but is a 

 separate generation producing spores; the spores ger- 

 minate and form a thread-like protonema, from which 

 the " moss-plant" arises. In our modern terminology, 

 there is an alternation between a sexual gametophyte 

 and an asexual sporophyte. But although the general 

 idea is clear enough now, it has had an intricate his- 

 tory, and there are still many unsolved problems. In 

 fact, as Dr. Scott has said, it remains "the greatest 

 mystery in the morphology of plants". From a scho- 

 larly historical sketch by Dr. W. H. Lang I have 

 selected the following notes on the development of the 

 idea : 



At first, the only alternation recognized in plants was 

 the alternation between vegetative shoots and reproduc- 

 tive shoots or flowers, which is a different question. In 

 l8t)i came Hofmeister's monumental work. In 1856 



