HELIOTROPISM 



461 



hand take up their positions partly passively, owing to the bending of the stem, 

 partly actively, so that their upper surfaces are exposed to the light, i. e. they 

 are plagioheliotropic. Positive heliotropism is a common phenomenon among 

 the shoots of the higher plants, especially in the seedling condition ; it occurs 

 in orthotropic leaves also, such as one meets with in the seedlings of many 

 Monocotyledons. It is by no means limited to green plant organs, for it is to 

 be met with in many Fungi. Thus the stalks of the fruits of Peziza fukeliana 

 and of Coprinus, the perithecia of many Pyrenomycetes and the unicellular 

 sporangiophores of Phycomyces, Miicor, and Pilobolus all bend towards the 

 light. A certain small number of roots also, e.g. those of Allium sativum, are 

 positively heliotropic. 



We are not nearly so well acquainted with the process of positive helio- 

 tropic curvature as we are with geotropic movements. The little we do know 

 tends to show that heliotropic does not differ fundamentally from geotropic 

 curvature. The comparison comes out first of all in the fact that — apart from 

 leaf articulations — the curvature is due to unequal growth of opposite sides, 

 and that apparently growth is retarded on the concave side and accelerated 

 on the convex; at the same time the median zone, 

 frequently at least, grows on at a uniform rate during 

 the curving. Generally speaking the first beginning of 

 the curving is manifested at the zone of most vigorous 

 growth in heliotropism also, which then proceeds basi- 

 petally, and becomes stationary in the last segment 

 capable of growth, while the apex straightens again. 

 We shall meet with exceptions to this behaviour later. 



In days gone by positive heliotropic curvature 

 was explained in a very simple and to a certain extent 

 purely mechanical way (De Candolle, 1832). The 

 explanation was based on the fact that growth in 

 length in many organs was retarded by illumination 

 and accelerated by shading. Hence, if an organ is more 

 brilliantly illuminated on one side than on the other 

 the former must remain shorter and become in conse- 

 quence concave, and so the organ must exhibit positive 

 heliotropic curvature. This hypothesis was, however, 

 overthrown by closer study of negatively heliotropic 

 organs. Negative heliotropism occurs in many terres- 

 trial roots, but most of all in aerial roots, in many tendrils, the hypocotyl 

 of the mistletoe, and, among unicellular structures, in the roothairs of 

 ferns and liverworts, &c. These organs all grow more rapidly in darkness 

 than in light, and on being illuminated on one side ought to exhibit the 

 same curving as is exhibited by positively heliotropic organs. Since they 

 do not do so, and since the illuminated side grows more rapidly and becornes 

 convex, we may fairly conclude that heliotropic curvature does not arise 

 directly from a difference in the amount of illumination on opposite sides of 

 the organ. As was the case in geotropic curvature, so here also we have to do 

 with a uniform reaction of the entire plant to an external stimulus. 



In order to prove this view of the case completely it has to be shown that 

 one and the same organ, may, according to the conditions, react positively 

 or negatively. Evidence of the most convincing character to this effect has 

 been brought forward by Berthold (1882) in certain marine Algae, which were 

 positively heliotropic in weak light and negatively heliotropic in brighter light. 

 Stahl {1880) has observed the same phenomenon in Vaucheria, and Oltmanns 

 (1897) has advanced exact experimental proof of a similar nature in the case 

 of Phycomyces, by allowing the fungus to grow at varying distances from an 

 electric arc lamp. Half an hour after the commencement of the experiment 



Fig. 145. Seedling of&'Mii/'/.S' 

 alba. The arrows mark the 

 incidence of the light. The 

 stem is positively, the root 

 negatively heliotropic. After 

 Sachs (1882). 



