252 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



when these shoot out in the most different directions and also when they 

 appear in the dark. In Kny's experiment the influence of light was not 

 excluded and the facts which have been stated above show that light must 

 be considered essential for anisophylly. It is probable also that cor- 

 relation plays a part. The lateral and under leaves of a twig of silver 

 fir can reach their ' fixed light-position ' quicker than do those of the upper 



side which have to undergo 

 a strong torsion. It is evi- 

 dent also that less plastic 

 material will flow towards 

 a strongly shaded branch 

 than to one strongly illu- 

 minated, and as a matter of 

 fact the total mass of the 

 leaves in the two is very 

 different. If we now assume 

 that shaded leaves are hin- 

 dered in their growth 1 , this 

 retardation must be more 

 felt in the upper leaves than 

 in the under which can draw 

 to themselves more rapidly 

 the available material, be- 

 cause their horizontal posi- 

 tion enables them to use 

 earlier the available light. 

 Gravity naturally acts 

 equally in both cases ; it 

 is however unable to bring 

 about a conspicuous aniso- 

 phylly. 



Another case in which 

 anisophylly is caused di- 

 rectly by light is that of 

 Lycopodium complanatum 

 (Fig. 125). In autumn I 

 placed in darkness the aerial parts of many plants of this species whilst 

 they grew in their natural localities by turning a pot over them. The 

 parts developed iJic next year were radial. We have already seen on 



FIG. 125. Lycopodium complanatum. Shoot which has pushed 

 out in darkness ; only the chief axis of this relatively lateral shoot 

 has pushed out, and the portion so developed has a radial con- 

 struction. Magnified. 



1 Weisse found that in shaded leaves of Acer platanoides the growth of the lamina and stalk 

 suffered diminution. See Weisse, Zur Kenntniss der Anisophyllie von Acer platanoides, in Ber. d. 

 deutsch. hot. Gesellsch. xiii. p. 379; also Wiesner, Photometrische Untersuchungen auf pflanzen- 

 physiologischem Gebiete, in Sitzungsb. d. Wiener Akad. d. Wiss. cii (1893), p. 291. 



