— 102 — 



common forra in May, but no conjugation was observée! until 

 the montli of June arrivée! (cf. also the behaviour of Zygnema 

 in 1906 and West, 1909, pp. 33, 34.) Nevertheless there is no 

 doubt tliat (in small pièces of water at least) tlie sexual process 

 of raost freshwater Algae generally coincides witli the attain- 

 ment or commencement of the maximum, and lasts until there 

 is a great diminution or complète disappearance of the species. 

 It must be borne in mind liowever that a marked maximum can 

 be attained by an Alga without anv resort to sexual reproduc- 

 tion, e. g. the broad species of Mougeotia in Barton's pond 

 (especially in 1906), Oedogonium in Abbot's pool (espeeially in 

 L905), Mougeotia'm Abbot's pool. etc. 



It might be argued that the setting in of sexual reproduction 

 with the arrivai of the maximum is proof of an inhérent ten- 

 dency in the Algae showing tins feature to advance to tins phase 

 of reproduction when growth lias reached a certain extent. 

 Rut in tins Une of argument two points are overlooked : firstlv, 

 as above mentioned, that a marked maximum cân be attained 

 without any sexual reproduction occurring; secondly, that the 

 attainment of a maximum in itself involves a cessation of much 

 further végétative growth or otherwise a still further increase 

 in the amount of the Alga would be recorded. In other words 

 we arrive at the same conclusion as Klebs {loc. cit.) that it is 

 the conditions which are unfavourable to much further végéta- 

 tive growth that call fortli the sexual reproductive process. 

 That thèse conditions are notnecessarily altogethér unfavourable 

 tothe further existence ofthe Alga is shown by the behaviourof 

 the species of Oedogonium, which may go on persisting in 

 some quanti ty after the sexual reproductive phase is over {cf. 

 also West 1909, pp. 33, 34). 



We hâve considérée! tins matter at some length owing to the 

 fact that since the publication of our first paper on Spirogyra 

 (Fritsch and Rich, 1907) in which we expressed the view that 

 sexual reproduction is due to the intensification of factors liable 

 to change in spring, two papers bave appeared which take up a 

 différent attitude. Brown (1908) ?1 reports on the periodicity 

 of a number of species and on the basis of thèse observations 

 concludes that Algae go on growing under steadv normal con- 



(1) Both Brown ami Copelaml overlooked our paper on Spirogyra in the 

 Armais ofBottmy for 1907. 



