EFFECTS OF LIGHT ON SIVARM-SPORES. 6ll 



series of special facts from the statements of the latter \ He investigated chiefly 

 the swarm-spores of Hcematococcus lacusin's, Uloihrix zona/a, Chceiomorpha cerea, 

 and of Ulva^ Botrydnim granulatum, Bryopsis, and other Algae, and especially those 

 of Chytridium, an Alga devoid of chlorophyll. Walz had already discovered in 1868, 

 and Cornu had confirmed the discovery, that the final formation and expulsion of the 

 sv^^arm-spores and antherozoids from their mother-cells is dependent not only upon a 

 proper temperature, but also upon a sufficient quantity of oxygen dissolved in the 

 water ; but light also promotes the birth of these small organs, as Thuret had 

 already discovered, so that, in fact, by keeping the Algae concerned in the dark and 

 then suddenly illuminating them, the moment of swarming can be artificially in- 

 duced. In the natural course of events, the dependence upon light referred to causes 

 the swarm-spores to escape from their receptacles and commence to swarm in the early 

 morning, as a rule at definite hours — i.e. when the light has attained a certain intensity. 



Strasburger describes the action of the light as follows : if particularly striking 

 phenomena are desired, it is well to begin the experiments with the swarm-spores of 

 Botrydhan gramdahim, the Alga figured on page 4 (Fig. 2). A preparation made one 

 day before-hand by sowing the spores, when brought from the dark into the light, 

 shows that at the moment of observation, all the swarm-spores are equally distributed 

 in the drop; nevertheless they instantly become directed with their anterior ends 

 towards the source of light, and now rush towards it in straight but otherwise fairly 

 parallel paths. After a few — usually ig to 2 — minutes, almost all the swarm-spores 

 have arrived at the illuminated side of the drop, and here swarm together in and 

 out. If the preparation is turned round so that its illuminated side is now away from 

 the source of light, all the swarm-spores which are still in motion instantly leave the 

 margin of the drop, now turned away from the source of light, and rush to what is 

 now its illuminated margin. 



For the sake of brevity the margin of the liquid which is turned towards 

 the source of light may be termed positive, and the opposite one negative. 



' If the swarm-spores of Ulothrix are selected for observation, the phenomenon 

 becomes in a certain sense still more striking. These also rush rapidly and in 

 almost straight paths to the positive margin of the drop, but it is only seldom that all 

 do this ; in most preparations, on the contrary, a larger or smaller portion of them 

 are seen to move equally rapidly in the opposite direction, and therefore towards the 

 negative margin. It is possible then to see a curious spectacle — the swarm-spores thus 

 rushing in opposite directions, and therefore with apparently double velocity, past one 

 another. If the preparation is turned round through 180°, the swarm-spores collected 

 at what was previously the positive side are seen to rush at once to the negative 

 side, and those previously collected at the negative side to rush to the positive side. 

 Arrived here, the swarm-spores move about in and out, keeping more or less close to 

 the margin, according to the preparation. One continually notices also, both at the 

 positive and at the negative side, individual swarm-spores suddenly abandon the margin 

 and rush directly through the drop to the opposite margin. Such an interchange 

 continually goes on between the two margins : nay, it not rarely occurs that single 

 swarm-spores just arrived from the opposite margin again return thence. Yet others 



• Strasburger, ' Wirkung des Lichtes und der JVän/ie auf Sch'wäriits/'orcii,'' Jena, 1S78. Stahl 

 in ' Verhandl. der med.-phys. Ges. Wzbg.,' 1879. 



R r 2 



