INFLUENCE OF EXTERNAL STIMULI. LIGHT 231 



mostly concrcscent with the shoot-axis, is quite dorsiventral and like a leaf 

 in the character of the difference in colour between the upper and the 

 under side. The illuminated side is the ' upper side.' The dorsiventrality 

 is reversible on the new growing parts. The influence of light is here 

 of more significance anatomically than organographically. 



Nuphar luteum however shows that dorsiventrality in an organo- 

 graphic sense can be 'induced ' through light. -In this plant the elongated 

 rhizome creeps in the mud and is densely clothed with leaf-scars. The 

 leaves arise radially in the apical stem-bud, but the insertions of the leaves 

 diverge much further from one another on the under side than they do upon 

 the upper side, thus giving us an approach to what occurs in many other 

 dorsiventral shoots 1 . The roots spring out of the under side, vegetative 

 lateral twigs shoot out from the flanks. If such a dorsiventral rhizome 

 of Nuphar is covered with earth it grows erect and radial until it again 

 reaches the light. The dorsiventrality is here caused by the light which 

 expresses itself in the displacement of the leaf-scars and in the position 

 of the roots. A similar influence of light is found in other Spermaphyta. 



In the plagiotropic climbing shoots of the ivy, for example, the roots 

 arise only on the shaded side, although in old plants one often finds the 

 whole surface of the shoot covered with roots ' 2 . The dorsiventrality is, 

 as Sachs has shown, reversible. Lepismium radicans and other climbing 

 kinds of Cacteae show the same features. It is evident that the roots 

 may be arranged in these plants all round the shoot, but their formation 

 is suppressed on the illuminated side. The researches of Sachs 3 have also 

 determined that the exclusion of light, or indeed a diminution of illumina- 

 tion, favours formation of roots, provided, of course, other conditions are 

 favourable. If plants of Phaseolus or Vicia Faba are cultivated in a moist 

 chamber in darkness, numerous adventitious roots shoot out from the etio- 

 lated stems at a considerable distance above the surface of the earth ; 

 these do not appear in presence of light. 



This influence of light upon the formation of roots in the higher plants 

 finds also a parallel amongst the lower ones. On the nodes of Chara 

 which usually bear no roots (' rhizoids '), these may be caused to develop 

 by keeping the shoots in darkness 4 . Investigations are still wanting 

 regarding the behaviour in this respect of the mosses ; we do know that 

 the shoots of many mosses are thickly clad with a felt of 'rhizoids' on 

 those parts which are exposed to the light. 



1 See p. 90. 



2 Possibly this is connected with the formation of a rind which light cannot or can only with 

 difficulty penetrate. 



3 Sachs, tiber den Einfluss des Tageslichtes iiber Neubildung und Entfaltung. Gesammelte Ab- 

 handlungen, i. p. 179 ; Id., Wirkung des Lichtes auf die Bluthenbildnng unter Vermittlung der Laub- 

 bliitter, ibid., p. 229. See also Vochting, Uber Organbildung im Pflanzenreich, i. p. 146. 



4 Richter, Uber Reaktionen der Charen gegen aussere Einfliissc, in Flora, Ixxviii (1894^, p. 399. 



