i896. NOTES AND COMMENTS. 293 



character by Lindau, while Hallier makes bristly-pollen the most 

 important feature in his limitation of the genus Ipomoea. 



Just as in zoology, there is a growing tendency among the 

 younger systematists to make use of a far wider range of characters, 

 the constancy of a character, and not its position, being the quality 

 that gives it classificatory value. Where botany appears to differ 

 from zoology is in the fact that so few changes are necessitated by 

 the more rigorous tests ; and this says much for the acumen of the 

 older workers, 



Alternation of Generations in Plants. 



Where must one seek the origin of the second generation ? In 

 the mosses, ferns, and higher plants there is a regular alternation of 

 a sexual and an asexual generation. In the Algae and Fungi there is 

 no such regularity; the same individual may bear reproductive organs 

 of both kinds. Is the neutral or asexual generation (the sporophyte) 

 something new which has been intercalated between two sexual 

 stages (oophytes), the latter alone being comparable with the indi- 

 vidual in the thallophyte ? or is the sporophyte also comparable to 

 such an individual, the two generations contrasting only in this — that 

 in the one reproduction depends absolutely on a sexual process, in 

 the other is purely asexual ? In a word, is the alternation antithetic 

 or homologous ? Bower, the chief exponent of the former theory, con- 

 ceives the sporophyte to have arisen as a mere group of spores in a 

 fertilised ovum. Hence the origin of its vegetative portion, whether 

 thallus or stem, leaf, and root, must be sought in the sterilisation of 

 this sporogenous tissue, that is to say, in a conversion of spore- 

 forming into vegetative cells. In the address we are discussing. 

 Dr. Scott preferred Pringsheim's view, which, by regarding the two 

 generations as homologous, obviates the necessity of a new birth. 

 On this view, the sporophyte is simply an individual precisely com- 

 parable with the oophyte, but it has acquired the habit of producing 

 only asexual spores, while the case of the oophyte is the converse. 

 As is well known, many green Algae, such as Ulothrix and CEdogonium, 

 can form asexual spores (zoospores) at any stage of their develop- 

 ment, but there is one stage, namely, on the germination of the 

 fertilised ovum (oospore), at which they are always formed. There 

 is no difference at all between the two sets of zoospores ; there is no 

 indication of anything new. In another green alga, Sphcsroplea, 

 zoospores are formed only on the germination of the oospore ; that is 

 to say, there is here as definite an alternation of a sexual generation 

 (the Spha^roplea plant) and an asexual (the oospore with its zoospores) 

 as in the mosses. 



Mosses and Ferns. 



Concerning the relations between mosses and ferns, Campbell's 

 AnthocerosAike ancestor for the latter does not find favour with Dr. 



