MOVEMENTS OF IRRITATION. 479 



somewhere in the neighbourhood of 28 C., but that of Zea at 

 33 G. The origination of the curvatures can thus not be regarded 

 as the result of difference in growth on the opposed sides of the 

 seedlings, directly dependent on the temperatures obtaining on 

 these two sides. If that were the case, then under the conditions 

 stated the Lepidium seedlings, e.g., must have curved straight 

 towards the source of heat, since the side remote from the plate 

 was exposed to a temperature more favourable for its growth than 

 the side turned towards the plate. The curvatures are, on the 

 contrary, to be regarded as irritation effects called forth by the heat 

 rays. Very searching investigation of the subject, however, is 

 still necessary. 2 



1 See Wortmann, Botan. Zeitung, 1883 and 1885. 



2 According to Jdnsson (Berichte d. Deutschen botan. Gesellschaft, Bd. 1, p. 

 512), roots also exhibit rheotropic properties. My investigations on the subject 

 are not yet complete. As regards the aerotropism (see Molisch, Berichte d. 

 Deutschen. botan. Gesellsch., Bd. 2, p. 160) and galvanotropism (see Elfving, 

 Botan. Zeitung, 1882) of roots, I have as yet made no experiments. 



183. Aerotropism and Chemotropism of Pollen Tubes and Fungal 



Hyphae. 



The germination of pollen grains, of which we shall frequently 

 have to speak, takes place best in sugar solutions (cane-sugar) of 

 particular concentration. The concentrations are first given in 

 the following table for some kinds of pollen : 



Alii um Vic toriale . .... 3 per cent. 

 Anthyllis vulneraria . . . . 15 

 Berberis vulgaris . . . . . 20 

 Colchicum autumnale . . . . 40 ,, 

 Digitalis ambigua . . . . 10 ,, 



Fritillari'a imperialis . . . . 10 ,, 

 Narcissus poetic us . . . 10 ,, 



Vincetoxicum officinale . . . 15 ,, 

 We now prepare sugar solutions with addition of ]-2 per cent, 

 of gelatine. This solution sets at 18 C. to a jelly. A drop of 

 the solution is placed on an]object-glass, and treated with pollen 

 grains, which we rapidly distribute very evenly by means of a 

 needle. We now lay on a cover-glass, taking care in doing so that 

 none of the fluid escapes beyond the edge of the cover-glass. The 

 formation of bubbles is also to be avoided. The slide cultures are 



