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 Vaucheria-iwhQ or internode of N Hella ; 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 Wolkoflf 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 <vohose growth 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 l.ea Mais, 
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 sati'vum gave, for example, the following results : — 
Day. 
Successive increments. 
In the dark. 
In diffuse light. 
ist 
195 mm. 
161 mm. 
2nd 
239 
153 
3rd 
250 
210 
4th 
126 
113 
5th 
113 
78 
In the 5 days 
923 mm. 
715 mm. 
3 H 2 
