6l2 LECTURE XXXV. 



remain stationary in the middle of their course, and then rush back to the point they 

 started from, eventually repeating the play to and fro for some time, like a pendulum.' 



The phenomena are usually less striking in the case of Hccmatococcus. In this 

 form, as well as in Ulothrix, however, Strasburger noticed that the smallest swarm- 

 spores exhibit the most rapid movement, and the colourless ones of Chytridium vorax 

 behaved exactly like those of Hccmatococcus., whereas other colourless swarm- spores, 

 such as those of the Saprolegniae, were not affected by light. 



One of the most important results of Strasburger's and Stahl's investigations is 

 the discovery that there are swarm-spores w-hich constantly rush towards the source 

 of light only, as well as such which move towards or from it, the ciliated end moving 

 forwards in the one or the other direction. In this matter the intensity of the light, 

 again, plays a very important part ; usually so that when the intensity of the light is 

 less the swarm-spores move towards the source of light, when stronger away from it. 

 This may be confirmed either by the observer moving his preparation further and 

 further from the window, or by remaining there and interpolating translucent shades. 

 If the light is strong, and the swarm-spores are collected at the negative margin of 

 the drop, a point at which the light is less intense is thus attained, at which they begin 

 to swim towards the positive margin. Another property of swarm-spores here comes 

 into action, which Strasburger designates their Phototonus — a property which even 

 with swarm-spores of the same species may vary with the time, and which consists in 

 that the reactions mentioned come into play only when the light reaches a higher or 

 lower intensity. If I understand Strasburger's statements correctly, however, this 

 phototonus is fundamentally nothing other than a lower or higher degree of irrita- 

 bility, of such a kind that swarmers which are adapted to low degrees of light are 

 sensitive to light of low intensity. 



The rapidity of the movement of swarm-spores, as Naegeli had already stated, 

 appears not to be influenced by light ; but Strasburger asserts that the swarmers move 

 in lines the more direct the more intense the ray of light which directs them is. 



Although the mechanics of the movement in all these cases, just as in the case 

 of the heliotropism of unicellular and multicellular plants, remains unknown to us, 

 it is nevertheless to be insisted upon that even in the latter, exactly similar degrees 

 of sensitiveness to light appear, and especially I have long ago established that 

 shoot-axes of the same plant — e. g. Tropccolum ??iajus — may be negatively heliotropic 

 in intense light, and positively heliotropic in feeble light. 



It should not be omitted that, according to Strasburger, the influence of light 

 in directing the movements of swarm-spores is the less the larger they are. To the 

 largest known swarm-spores in particular, he denies all irritability to light whatever, 

 as Thuret had already done also. Nevertheless there are exceptions : the small 

 yellow swarm- spores of Bryopsis appear not to be sensitive to light, whereas the 

 large green ones are very sensitive. 



With respect to the refrangibility, colour, or wave-lengths of the effective rays 

 of light. Strasburger here also confirmed the law which I had already established, 

 that the mechanical actions of light on plants are due chiefly or exclusively to the 

 highly refrangible, blue portion of the spectrum. When he allowed the light acting 

 on the swarm-spores to pass through a dark solution of ammoniacal copper oxide, 

 which transmits the blue and violet light, the swarm-spores reacted exactly as 



