— 105 — 



organs (1) . On the other hand with observations based onlyon 

 fortnightly samples, il is impossible to say whether in the inter- 

 vnl between the collection of two successive samples conditions 

 may not hâve arisen, which temporarily put a check on the 

 formation bî sexual organs and causeda rapid végétative pi opa- 

 gation of the form in question. Sucli oscillations are quite con- 

 ceivable and would afford ample explanation of the phenomenon 

 just referred to. 



Although our observations support those of Klebs in appor- 

 tioning to liglit the main rôle in stimulating the sexual repro- 

 ductive process, it is quite plain that tins is not the onlv factor 

 that cornes into play. In the first place it is probable that the 

 Alga must hâve attained to a certain degree of vigour, and tins 

 is probably the reason why most Algae do not reproduce sexually 

 until they hâve become well established. Apart from that, 

 température and concentration of the water are no doubt of 

 importance, although their exact value will probably be found 

 to vary much in différent cases (cf. also Klebs). It can only be 

 factors such as thèse that can account for the anomalies in repro- 

 duction referred to above (see especially, p. 101). 



In concluding our discussion of the sexual reproductive pro- 

 cess in Barton's pond, we may once more refer to the question 

 of the causes of subséquent germination of the resting spores 

 thus formed (cf. p. 97). There is plenty of évidence that such 

 spores can lie dormant even for several years before germination 

 occurs, and we hâve ourselves contributed some observations of 

 tins kind. In most cases however germination would appear 

 to occuralmost regularlyin the subséquent season andatapproxi- 

 mately the same time each year. In view of tins it is difficult 

 altogether to sliake ofi' a belief in an inhérent tendency of spores 

 to germinate after a certain resting period If tliis be the 

 case the exact time at wliich germination occurs evidentlv 

 dépends on outside conditions, and we must suppose that the 

 latter can be of such a nature as to make germination impos- 

 sible for one or more years. 



Neither Microspora nor Conferva were ever observed to 

 form anv definite resting-spores, but towards the end of their 

 periods of dominance large quantifies of food-reserves (starch in 



(1) It should he noticed that in thèse cas.s very l'ew sexual orcans are found 

 until the maximum is reached. 



