INFLUENCE OF EXTERNAL STIMULI. LIGHT 255 



excluded, and Frank is hardly warranted in his conclusion from this 

 experiment that the development of the under leaves to a greater extent 

 than the upper ones on the shoots of Acer obtusatum produced by buds in 

 darkness is an effect of gravity, because the anisophylly may be already 

 induced in the bud, as I have shown above to be the case in Aesculus and 

 Abies. In shoots of Abies canadensis the length attained by the upper 

 leaves in light was about 4-7 mm., that by the under leaves was 13 mm. ; in 

 the natural position when developed in the dark they were respectively 

 6-2 mm. and n-6 mm.; in the inverted position in the dark they were 

 7-2 mm. and 10-3 mm. From these figures Frank concluded that light 

 acted as well as gravity upon the anisophylly in this case. 



From what has been said it will be gathered that anisophylly is 

 a complex phenomenon associated with different factors, but that every- 

 where it may be induced by definite, chiefly external, factors. These can 

 directly influence the configuration, for example, in Abies pectinata and 

 Lycopodium Chamaecyparissus, or impress upon the vegetative point 

 of the bud definite peculiarities 1 , or the ' induction ' is, as in the case of 

 habitually anisophyllous plants, a permanent one. One may also assume 

 such an induction for those cases in which anisophylly appears to depend 

 entirely upon the position of the lateral shoots in relation to the mother-axis. 

 Further investigations are required for the full explanation of the question 

 investigations which must specially take into consideration the induction, pro- 

 bably appearing at a very early time, in the vegetative point of lateral shoots. 



Kolderup-Rosenvinge 2 has found in Centradenia floribunda that in 

 shoots fixed horizontally in an inverted position the anisophylly is in- 

 verted, that is to say, the larger leaves stand upon the side which was 

 formerly uppermost. It is a fact that the anisophylly of this plant is 

 very markedly influenced by the position of the shoot to the horizontal, 

 and hence vertically growing shoots hardly exhibit it at all, whilst the 

 horizontal ones do so in considerable amount. The experiments upon 

 inversion which have been mentioned do not however prove that gravity 

 is the critical factor in anisophylly, inasmuch as light may likewise in- 

 fluence the shoot differently in the horizontal position and in the vertical. 



(d) CHANGE OF FUNCTION THROUGH LIGHT. 



The cases of the qualitative influence of light upon the formation of 

 organs which have now to be dealt with cannot be sharply separated 

 of course from those which have just been referred to, but it is interesting 

 to deal with them specially. 



1 According to Frank's experiment they appear to act chiefly on the earliest stages of the primordia 

 of the leaves. 



2 Kolderup-Rosenvinge, L'Organisation polaire et dorsiventrale des Plantes, in Revue generale de 

 Botanique, i. p. 130. 



