254 SCIEXCE PROGRESS. 



made to brin^" into the sphere of actual experimental enquiry 

 a subject which has baffled physiologists for a long time. 



If we attempt to see what light this throws on the 

 question of alternation of generations the following remarks 

 occur to one : — 



Among the Thallophytes we meet with sporo- 

 genous tissues giving rise to asexual spores, and with 

 oospores developed sexually, but if we attempt to read 

 into these and their sequence the dogma of alternation of 

 genertions, some quaint difficulties arise. 



One of the most convincinQ; cases to manv minds is 

 that of such Algai as Qidogonuuji, Colcochoetc, etc., where 

 if we call the plant or portion w^hich bears the sexual- 

 organs a gametophyte generation, we have to face the 

 difficulty that it may at the same time bear asexual spores. 

 The attempt to get over this by terming asexual spores 

 borne by the gametophyte gonidia, and reserving the term 

 spore for bodies indistinguishable from these go nidia by any 

 morphological or physiological character ivhatsoever beyond 

 their origin from a so-called sporophyte,^ carries its own 

 refutation. 



Now the principal interest of Klebs' work centres in his 

 proof that asexual zoospores as well as sexual organs can be 

 called forth or suppressed altogether, practically at will, in 

 any part of the life-cycle of even so highly developed an 

 Alga as CEdogonium, or of so specialised a Fungus as 

 Eurotiu?n, by merely altering the conditions of the environ- 

 ment. To speak of alternate generations in such cases 

 seems impossible : the assumed mark of a generation is 

 here a matter of conditions, simply, and cannot be regarded 

 as a morphological necessity coming into evidence at just 

 such a place and such a time in the life of the plant as is 

 demanded by some special structure and working of the 

 organisation from the beginning. 



Few Algae have been more thoroughly studied than 

 Ulothrix zonata ; but Klebs shows that it depends on con- 

 ditions whether a filament forms (i) Zoospores with four 



^ A point already insisted upon by Scott also in his address to Section 

 K, British Association, Liverpool 1896, p. 7. 



