146 HISTORY OF BOTANY 



the function of nutrition ; a higher morphological 

 differentiation of parts followed, and a more clear dis- 

 tinction between the organs which were to supply the 

 nutriment (stem, leaves, roots) and the parts devoted 

 to the formation of spores (sporangia), this for the 

 first time stamped the sporophyte with a character of 

 independence and permanence." 



The general survey carried out by Bower and others 

 brought into the foreground three distinct but related 

 problems : (i) The relationship of the so-called homo- 

 logous alternation of generations in the Thallophyta 

 (combined, as it is in some forms, with the somewhat 

 confusing phenomena of the appearance of the carposporic 

 stage) with the pronounced antithetic alternation of 

 gametophyte and sporocarp in the Bryophyta ; (2) the 

 point of origin of the Pteridophytic hne with its in- 

 dependent and highly elaborate sporophyte and greatly 

 reduced gametophyte ; (3) the homologies between the 

 Pteridophytic reproductive organs and alternation and 

 the condensed and concealed reproductive structures 

 in the Phanerogams. 



(i) In the Thallophyta, you will remember, it was 

 recognised that a sexual stage is, as a rule, succeeded by 

 several asexual generations, and that the cycle is finally 

 closed by a sexual stage once more. It had been noted, 

 however, that plants producing sexual organs might give 

 rise to asexual organs also ; indeed, long after Celakowski's 

 time it was shown that the formation of carpogonia, 

 antheridia, and tetragonidia on the same plant, and 

 even in the same branch of such a Red Seaweed as Poly- 

 siphonia, was by no means a rare occurrence. In 1896 

 Klebs pubHshed the results of twenty years' experimenta- 

 tion, which tended to show that sexual, asexual, and 

 neutral conditions could be induced in many of the 

 lower Thallophyta, both green and non-green, by changing 

 the composition of the culture media, with or without 

 altering the thermal, photic, or other environmental 



