232 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



The formation of other subterranean organs besides roots is favoured by 

 the absence of light, and in this way the effect of light in their disposition 

 is observable. Thus light retards the formation of tubers on the potato l ; 

 darkness favours it. Plants in which the formation of stolons has been 

 hindered and from which the possibility is thus removed of the formation 

 of underground tubers may be caused to produce tubers near the apex 

 of their aerial shoots by excluding light from them. Gravity also seems 

 to exercise an influence upon the formation of tubers in this case inasmuch 

 as the tuberous shoots appear on the parts of the chief axis which are 

 turned to the soil 2 . At any rate under normal conditions, light, by 

 retarding the formation of tubers on the aerial parts, has a directive 

 influence and favours the formation of tubers on the subterranean 

 stolons. 



Roots and potato-tubers are organs which arc ' attuned ' to darkness 



and whose appearance there- 

 fore is retarded by illumination. 

 The converse case, namely, the 

 arrestment through feeble illu- 

 mination of organs ' attuned ' 

 to high light-intensities, fre- 

 quently occurs. When speaking 

 of relationships of symmetry 3 , 

 I showed that in spruce the 

 plagiotropous lateral shoots 

 are originally radially branched, 

 and that on account of the 

 want of light the shoots upon 

 their upper side are suppressed ; 

 in other needle-leaved trees the suppression of these on the under side also 

 takes place. A like feature is observable in broad-leaved trees, and has 

 a marked influence upon the habit of the whole plant. In Salix incana 4 

 the leaf-buds develop only upon the side of the shoot which is well 

 illuminated, in this case the upper side ; in Populus pyramidalis it is the 

 buds which stand upon the outer side only of the straight erect shoots 

 that are developed. 



Light has a share also in the formation of the plagiotropous shoot- 

 system of the mosses, which branches in one plane 5 . There are transitions 

 from the radially branched ordinary shoots to the feathered ones of 



FlG. 113. Tliuidium abietinuin. Transverse section of a 

 chief shoot. The illuminated side of the axis is flattened, 

 right and left arise two lateral shoots. On the shaded side is 

 the primordium (apical cell) of an undeveloped shoot. Half 

 diagrammatic copy of a drawing by Kienitz-Gerloff. 



1 Vochting, IJber die Bildung der Knollen, in Bibliotheca botanica, Heft 4 (1887). 



2 For details see Vochting, 1. c., p. 39. 3 See p. 94. 



4 See Wiesner, Untersuchungen iiber den Lichtgenuss der Pflanzen mit Ritcksicht auf die Vegetation 

 von Wien, Cairo, und Buitenzorg, in Sitzungsber. der Wiener Akad. d. Wissensch., civ (1895), p. 685. 



5 See p. 69, and Figs. 27, 28, 29. 



