250 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



The phenomena of anisophylly are in the main, as we have already 

 seen, connected with the provision of a favourable surface for assimila- 

 tion, and we have therefore next to speak of it in its dependence upon 

 external factors, so far as we know them. 



(c) ANISOPHYLLY IN RELATION TO LIGHT. 



In the second section, I have discussed the features of anisophylly in 

 detail, and shown that, as Herbert Spencer first recognized, the advantage 

 of anisophylly lies in the favourable surface for assimilation it provides. This 

 does not mean that light must be the determining factor for the occurrence 

 of anisophylly ; but I showed some years ago 1 , in examples from the 

 silver fir, that anisophylly is strongly influenced by light. The plagio- 

 tropous lateral shoots of this tree are, as is well known, dorsiventral. ' This 

 appears both in the position and in the construction of the needle-leaves. 

 The former varies .... according to the illumination ; in the lower twigs 

 of trees standing in close wood or of young examples growing in the shade 

 of older ones, the whole needle-leaves are "pectinated "' 2 , through twisting of 

 the leaf-base, and have their green upper side turned upwards to the light, 

 their white under side turned downwards away from the light. Such 

 shoots behave then like a thallus of Marchantia they possess an upper 

 side constructed differently from the under side. The influence of light 

 shows itself also in the relationship in size of the leaves the leaves on the 

 upper side are distinctly shorter than those on the under side. The 

 following measurements of length of leaf will illustrate this : 



1. Leaf upon the under side of a twig: this leaf turned its upper 

 side upwards without torsion 16 mm. 



2. Next following leaf approaching the flank of the dorsal side 

 -10-5 mm. 



3. Next leaf inserted entirely upon the upper side 8 mm. 



4. Next leaf inserted entirely upon the under side 18 mm. 

 There is thus a difference amounting to more than double their 



length between the shortest and the longest leaves, and the smallest are 

 those which stand furthest apart upon the upper side, the largest are those 

 standing upon the shaded side but which really assume a lateral position 

 on the shoot. The leaves on the erect chief shoot are on the other hand 

 all equal in size, and compared with them those which stand upon the 

 illuminated side of the twigs have suffered a restriction in their develop- 

 ment. In strong illumination in the open the needles on the twigs are 



1 Goebel, Vergleichende Entwicklungsgeschichte der Pflanzenorgane, p. 146. See also Goebel, 

 Uber einige Falle von habitueller Anisophyllie in Botan. Zeitung, 1880, p. 839, where I deal with 

 the relationship of light to plagiotropous growth. 



2 The character of the foliage of the silver fir expressed in the specific name Abies pectinata. 



