— 99 — 



<>n périodicité (West, 1909, pp. 85-86), dealing however with the 

 alga] flora of a pièce of water in A.ustralia, shows anyhow in 

 the first hall* of Lhe year a very similar periodic fcrend to thaï 

 recorded in Barton's pond and Abbot's Pool. In larger pièces 

 of water in our paris owîng to the more graduai rise of tempe- 

 rature one may expect the dominance of ihe différent forms to 

 oceur at a later date than in small ponds {cf. Brunnthaler, 1907. 

 p. 190). 



F. — General considération of reproduction 

 in Barton's pond. 



With the nnmerous changes in the annual cycle of the algal 

 dora there must naturally be coupled an abuudance of reproduc- 

 tive processes, some leading merely to the maximum of a given 

 form (végétative propagation and zoospore production), others 

 to the development of resting spores (sexual or asexual). It is 

 onlv the la t ter type of reproduction that can be readily observée! 

 in a séries of samples such as that on which the présent paper 

 is built up, since végétative propagation or the production of 

 zoospores may hâve taken place abundantly between the collect- 

 ion of two consécutive samples without leaving any indication 

 except the increase of the form or forms in cniestion. For this 

 reason the folio wing remarks are almost wholly concerne! 1 with 

 the production of resting spores by the members oï the algal 

 flora of the pond. Attention may however be drawn to the l'act 

 that our preceding considérations (in sections D and E) hâve 

 really dealt with the conditions favouring or adverse to végéta- 

 tive propagation in the différent forms, since a maximum is only 

 attainable if reproduction of this type takes place; possibly the 

 same conditions are also in part more or less favourable to zoo- 

 spore-production. 



The production of resting-spores in Barton's pond lias been 

 observée! : in the four species of Spirogyra, in Zygnema and 

 Mougeotia parvula; in Clostrrium Kûtzingii and in Stau- 

 rastrum; in the four species of Oedogonium and in Bulbo- 

 chaete ; in Coleochaete ; in Anabaena. In ail, except the last- 

 named case, the spores are produced as the resuit of a sexual 

 process (1 >. This sexual reproduction may first of ail be consi- 

 dered for the pond as a wholé. 



(1) There is nothint: however to indicate that the conditions governin^ spore- 

 tbrmation in Anabaena are in any marked way ditï'erent i'rom those causinu' the 

 formation of the sexually produced spores. 



