INFLUENCE OF EXTERNAL STIMULI. LIGHT* 



227 



B. INFLUENCE OF LIGHT. 



Light has a much more powerful influence upon the relationships 

 of configuration of plants than has gravity. Here we have not to deal 

 with processes standing in direct connexion with assimilation, but with 

 specific stimulation-effects which at present we cannot satisfactorily 

 explain. In support of this we note that the effects of light are not 

 limited to plants with chlorophyll, that its influence cannot be replaced 

 by the addition of organic food-material, and that it is not the light- 

 rays specially concerned in assimilation which are operative as the 

 stimulus but those of the more refrangible portion of the spectrum. 



I. DIRECTIVE INFLUENCE OF LIGHT. 



Light determines in a number of cases of dorsiventral organs which 

 side shall be dorsal and which shall be ventral. Different plants react 

 differently in this respect. In some, like 

 the prothalli of Polypodiaceae, the dorsi- 

 ventrality is reversible at any moment if 

 the side of illumination be changed ; in 

 others the dorsiventrality once constituted 

 is permanent and cannot be reversed. 

 I may give some illustrations. 



The longest known example is that 

 of the plants which are developed from 

 the gemmae of Marchantia and Lunularia. 

 The gemmae which arise in the gernmae- 

 cups, and appear primarily as vertical cell- 

 bodies, are alike in formation on both 

 sides and remain so. They have however 

 at two opposite points on the margin 

 the primordium of a vegetative point, 

 out of which a new dorsiventral thallus 

 may arise if the conditions be favourable 

 (Fig. 112). This thallus has upon its dorsal surface which is normally 

 directed upwards characteristic assimilation-tissue, whilst upon the ventral 

 side hair-roots and scales which serve for the protection of the vegetative 

 point occur. Mirbel l long ago recognized that external factors determine 

 the side which is to be dorsal and that which is to be ventral ; Pfeffer 2 made 



FIG. 112. Marchantia polymorpha. A C 

 gemmae in different stages of development ; 

 st stalk-cell D mature gemma in surface 

 view, on each side is seen a vegetative point 

 which can grow out into a new thallus. E 

 transverse section of D through the lateral 

 vegetative points ; x point at which stalk 

 was attached; o oil-cells; ; cells distinguished 

 by their size and contents out of which the 

 hair-roots develop. Lehrb. 



1 Mirbel, Recherches anatomiques et physiologiques sur le Marchantia polymorpha, in Mem. de 

 1'Acad. des Sciences de 1'Inst. de France, 1835. 



2 Pfeffer, Studien liber Symmetrie und spezifische '\Yachstumsursachen, in Arbeiten des botan. 

 Instituts in Wiirzburg, i. p. 77. See also Zimmermann, Uber die Eimvirkvmg des Lichtes auf den 

 Marchantiathallus, ibid. ii. p. 665. 



Q 2 



