EFFECTS IN THE SPECTRUM. 697 



tropic effects; the greatest stimulating force always resides at the boundary between 

 violet and ultra-violet, proceeding thence the effect sinks gradually until the green, 

 and suddenly disappears in the yellow part of the spectrum, thence onwards it begins 

 again in the orange and ascends up to the ultra-red, where a maximum is reached 

 which is smaller than the one first mentioned. In the yellow portion of the 

 spectrum there is, according to Wiesner, not only no heliotropic effect whatever 

 to be noticed, but it even appears that it diminishes the influence of the orange 

 and yellow rays. 



The observations behind coloured screens then, do not quite agree with those 

 in the objective spectrum ; but I have already pointed out in my ' Handbook ' 

 (1865, p. 42) that there are certain sources of error connected with the latter mode of 

 observation, and we may be sure that more has yet to be done in the matter. 



Finally we come to the question in what manner do gravity and the rays 

 of light act on the processes of growth, that they are able to produce geotropic 

 and heliotropic curvatures ? In spite of many hypotheses, however, as good as 

 nothing is known about the matter. That when curvature takes place the 

 turgescence of multicellular organs increases on the side which becomes convex 

 is practically obvious; the question is simply why this takes place when gravity 

 or a ray of light meets a geotropic or heliotropic organ transversely or obliquely 

 to its longitudinal axis, and why the effect ceases as soon as that axis has assumed 

 the direction of gravity or of the ray of light. The question, how^ever, only obtains 

 a perfectly clear and strict form if the geotropic and heliotropic curvatures of the 

 non-cellular plants also are taken into consideration, where the positive and negative 

 curvatures of simple vesicles are concerned, and where there can be no question of 

 a difference of turgescence on the convex and concave sides of the organ. 



It would be a poor makeshift to try to explain the geotropic and heliotropic pro- 

 cesses in mukicellular plants by themselves, and to seek another explanation in the case 

 of the non-cellular plants, and this for a very simple reason. If we take the case of 

 a single cell in the primary stem or primary root of the Mustard plant considered 

 above, which cell is situated in a curved portion, then this one cell behaves exactly 

 like a curved non-cellular utricle of Mi/cor, Vaucheria, or any other similar structure. 



