ZOOLOGY AND BOTANY, MICKOSCOPY. ETO. 91 



spores of Ulotlirix tenuis, the latter with a slight difference. Swarm- 

 spores of PolypJiagus Euglense and a Bodo (?) appeared to be quite 

 indifferent to gravitation ; and diatoms and Oscillariaceae appear to 

 be neither geotropic nor aerotropic. 



The most complete series of experiments made were those with 

 regard to the heliotropic movements of desmids. He found, in all the 

 species examined, that when subjected to diffused light on all sides, the 

 longer axis placed itself in such a direction that one end of the cell 

 rested on the substratum, while the other placed itself in such a position 

 that the angle of elevation was between 30° and 50°. The free end of 

 the cell moves about with a motion which the author believes to depend 

 on nutation ; but in diffused daylight there is no definite direction of the 

 axis nor of the movement. In Pleurotaenium nodulosum and carinatum he 

 found a nutating movement of the free end of the cell, the direction 

 of the axis changing with the direction of the incident rays of light. 

 Gosmariuni Meneghinii and Closterium striolatum exhibit also a definite 

 direction of the axis with very weak light ; but this was not the case 

 with the other species examined. The direction of the movement in 

 Pleurotsenium is towards the light. Desmids are, therefore, positively 

 heliotropic. 



When swarm-spores move forwards with the portion which bears the 

 cilia in front, this, the author believes, is another illustration of the same 

 law. When light is allowed to fall on them on one side, the swarm- 

 spores place themselves with their longer axis in the direction of the 

 incident light, and with the cilia turned either towards or away from 

 the source of light, and then either positive or negative heliotropic 

 movement takes place ; the author finds in these phenomena an exact 

 analogue of the heliotropic or geotropic curvatures of the higher 

 organisms. 



The angle which the alga makes with the substratum varies with the 

 species ; and this he terms the " special angle " (Eigenwinkel) of the 

 species, and asserts that it is not affected by the nature of the sub- 

 stratum. He is able to reconcile with the above theory the statement 

 of Stahl that, when moving away from the light, the axis of the alga is 

 nearly or quite at rightangles to that of the rays of light, and he 

 confirms Stahl's statement that when the illumination is strong, PleurO' 

 tsenium exhibits striking negative heliotropism. 



The author has at present been unable to determine whether desmids 

 are also geotropic. 



Photo-position of Leaves.* — Herr H. Vochting calls attention to 

 some old observations of Eatchinsky that, in Malva rotundifolia and 

 in some allied species, at night the leaf-stalk makes a more acute angle 

 with the leaf than in the day ; and that in the daytime the leaves 

 follow the course of the sun in such a way that the surface of 

 the lamina is always at right angles to the incident rays of light, 

 whether the radiation be more or less intense. Soon after sunset 

 they take up their nocturnal position. These changes in position 

 Vochting states to be determined entirely by light, causing the morpho- 

 logical upper side only to be illuminated ; the geotropism of the lamina 

 and its weight have no influence on these movements. While the 

 lamina of the leaf shows neither epinasty nor hyponasty, the lower 



* Bot. Ztg., xlvi. (1888) pp. 505-14, 517-27, 533-41, 549-60 (1 pi.). 



