IRRITABILITY. 523 



continuous darkness on zoospores appears to be that it prolongs 

 their period of motility: instead of coming to rest and germi- 

 nating, they continue to swim about until they die. Strasbur- 

 ger observed that the zoospores of Ulothrix sonata continued 

 to move for about three days, and those of Hczinatococcus 

 lacustris for over a fortnight, in darkness. But there is a case 

 to which allusion was made in a previous lecture (p. 299), in 

 which the tonic influence of light is an essential condition, 

 namely, the movement of Engelmann's Bacterium photometri- 

 cum. This is a case of very great interest. We have here, 

 namely, a motile organism which can only move in the 

 presence of light, or in other words, when it is in a phototonic 

 condition, a condition which we have found to be essential to 

 the growth of leaves (p. 380). When the Bacteria are exposed 

 to light, movement does not at once begin, but there is a 

 "latent period," which is the shorter, the more intense the light. 

 Similarly when the Bacteria are placed in darkness, the move- 

 ment does not cease abruptly, but gradually becomes less and 

 less active : exposure to light produces a well-marked after- 

 effect, the duration of which is the longer, the longer, and 

 especially the more intense, the previous illumination. When 

 exposed for some time to bright light of constant intensity, 

 the Bacteria come to rest, especially in the absence of an 

 adequate supply of free oxygen : when they have come to rest, 

 they can be readily stimulated to renewed movement by a 

 sudden and considerable variation in the intensity of the light, 

 provided that the period of rest has not been longer than a few 

 minutes; when, however, the Bacteria have been at rest, exposed 

 to light, for some hours, they are no longer irritable by varia- 

 tions in the intensity of the light. Engelmann has ascertained 

 that, of the rays of the spectrum, the dark ultra-red are those 

 which are most active in inducing the phototonic condition ; 

 next in order come the orange, yellow, blue, and visible red. 

 The only other case which at all resembles this of the Bacte- 

 rium, is that of the Oscillatorias, which, according to Famint- 

 zin, move less actively in darkness than in light. On the 

 other hand, instances are on record of a diminution of motility 

 in consequence of exposure to light; this has been observed 



