REPRODUCTION. 193 



species would therefore be gradually reduced to such small size as 

 to render existence impossible. Extinction due to decrease in size 

 is avoided by two methods. The first, after certain pauses, fully 

 restores the original size of the individual. This is accomplished 

 as follows : From time to time two small individuals unite with 

 the escape and fusion of the cell- contents. This conjugation 

 gives rise to cm exceptionally large individual, sometimes two. 

 The second method is, so to speak, corrective, in that it tends to re- 

 tard the decrease in size. OTTO MULLEB ' has discovered the fol- 

 lowing law of development : The smaller of the two daughter -cells 

 requires twice as long a period for the next division as the larger 

 cell. The large spores formed by conjugation are called auxospores, 

 and are sometimes formed from a single individual (without conju- 

 gation). 



Protococcoideas,. Either vegetative reproduction by division, 

 or sexual reproduction by the union of swarming gametes which 

 differ in external appearance. 



Confervoidece. Asexual reproduction by means of swarm- 

 spores ; sexual reproduction by conjugation ( Ulothrix). Anther- 

 ids and oogonia are formed in some cases (Oedogonium, Bid- 

 bochaetce). From PRINGSHEIM'S classical investigations of the alga 

 Oedogonium I select the following : The oospore formed during the 

 previous vegetative season produces four swarm-spores which de- 

 velop into new filamentous algae. Swarm-spores are also formed 

 from the vegetative algal threads. The oospore is the result of the 

 fertilization of the egg- cell by means of the sperrnatozoids which 

 enter through an opening in the covering of the oogonium. The 

 sperrnatozoids are produced in two ways : either directly from the 

 cells of an ordinary filament, or from a small few-celled male plant 

 (Zwergmannchen). The latter is developed from an " andro- 

 spore," a peculiar swarm-spore which, after liberation and swarm- 

 ing, comes to rest and, attaching itself in some suitable spot, devel- 

 ops a few small cells. From these cells the spermatozoids, which 

 finally escape and fertilize the egg-cell, are formed. 



Among Characece and Vaucheriacece there occurs a sexual 

 propagation, besides sexual reproduction which is highly specialized 

 in the former group. Among Characece propagation is accom- 



From the study of Melosira arenaria, Ber. cl. Deutsch. Bot. Ges. 5 I, 1883 



