ACTION OF LIGHT ON GROWTH IN LENGTH. 835 



extensibility of the cell-wall under the influence of the pressure of the sap on 

 the side exposed to the strongest light. This hypothesis would be confirmed by 

 Kraus's observations, according to which the cuticularisation of the epidermis as well 

 as the thickening of the walls of the cortical and bast-cells is in fact imperfect in 

 etiolated internodes, and the extensibility of these cell-walls consequently increased 

 by the want of light. This explanation would apply not only in the case of the 

 shaded side of a multicellular internode which curves towards the light, but also 

 in that of a Vauchen'a-iuhe or internode of Nitella ; since it may be supposed 

 that the wall is in the first place more strongly thickened on the side exposed 

 to light and hence becomes less extensible, and therefore yields less to the pressure 

 of the sap, and, in consequence, grows more slowly. We have at present no 

 observations on heliotropic unicellular filaments. 



If it be proved, as the recent researches of Wolkoff give ground for believing, 

 that the negative heliotropism of organs which contain chlorophyll depends as 

 litde as that of roots on the stronger power of assimilation possessed by the 

 side exposed to the source of light, it must be assumed that all the actions 

 which have been mentioned as possible in one direction may take place also in an 

 opposite direction ; and this will show the great difficulty of the investigation. 



A complete account of the mode in which growth depends on light is scarcely 

 possible at present ; what has now been said will call the attention of the reader to the 

 most important questions involved in the investigation. It may be desirable however to 

 collect some of the more important facts at present known, and to add some critical 

 remarks. 



(a) Organs nuhose gronvth is retarded by light. To take first the case of those inter- 

 nodes (including, according to Hofmeister, the unicellular ones of Nitella) which, when 

 the light is unequal on the two sides, curve so that the side facing the source of light is 

 concave while the other side is convex, or in other words are positively heliotropic. 

 These exhibit a periodicity in their longitudinal growth corresponding to the alternation 

 of day and night, when the temperature is sufficiently constant. The growth is more 

 rapid from evening to morning, and less so from morning to evening. Both these facts 

 are consistent with the phenomenon that the same internodes grow longer, and often 

 considerably so, in permanent darkness than they would under normal conditions. 

 These three results lead naturally to the conclusion that it is the direct action of light 

 (and only in fact of its more refrangible rays, see Sect. 8) which retards the growth of 

 these internodes. In the case also of positively heliotropic roots (as those of Zea Maisj 

 Lemna, Cucurbita, Pistia, &c.), it may be supposed that if exposed to daylight they 

 would exhibit the same alternation as internodes ; but this is not yet fully established. 

 WolkofF has, on the other hand, already shown in the case of some roots, grown in 

 water behind a transparent glass plate, that they grow more quickly in permanent 

 darkness than under the alternation of day and night. Twelve primary roots of seed- 

 lings of Pisum sativum gave, for example, the following results : — 



Day. Successive increments. 



In the dark. In diffuse light. 



161 mm. 



153 

 210 



»I3 



78 



715 mm. 



