832 
MECHANICS OF GROWTH. 
Sect. 20.— Action of Light on Growth in Length \ — Heliotropism ^. 
Since we shall now pay exclusive attention to the questions whether and in what 
way light promotes or retards quantitatively the superficial growth of the cell-wall, 
we may for the time leave entirely out of consideration those cases where it changes 
^ A, P. De CandoUe, Physiologie vegetale, Paris 1832, vol. III. p. 1079. — Sachs, Bot. Zeitg. 
1863, Supplement, and 1865, P- ^^1- — I^itto, Experimental-Physiologie, Sect 15. — Hofmeister, Lehre 
von der Pflanzenzelle, Sect. 36. — Kraus, Jahrb. für wissensch. Bot. vol. VII. p. 209 et seq. — Batalin, 
Bot. Zeitg. 1871, No. 40. 
2 [H. Müller (Thurgau), Ueb. Heliotropismus, Flora, 1876; Wiesner, Die Heliotropischen 
Erscheinungen, Denkschr. d. k. k. Akad. in Wien, 1878, 1880; Darwin, The Movements of Plants, 
1880. 
A brief account of Wiesner's conclusions may be found useful ; they are as follows : — 
I. Influence of the Intensity of Light. 
T. The maximum of heliotropic effect is produced by a certain intensity of light : increase or 
decrease of intensity diminishes the heliotropic effect until it is no longer produced. The optimum 
intensity varies in different plants. 
2. The upper limit of intensity is either greater or less than that degree of intensity at which 
the parts of plants in question can grow at all : this depends upon the relative sensitiveness of the 
plants. 
3. Hence it appears that sunlight may absolutely arrest growth : young stems are protected by 
their strong negative geotropism from the action of sunlight. 
4. The degree of intensity at which heliotropism ceases corresponds, doubtless, to the intensity 
at which the plant no longer reacts by growth ; an intensity which affects the plant no more than 
complete darkness. 
II. Influence of Refrangihility. 
1. Not only do the rays of high refrangihility possess heliotropic power, but those also of lower 
refrangihility: it is possessed by all rays from the ultra-red to the ultra-violet except the yellow rays. 
2. The most marked effects are produced by the rays at the junction of the violet and ultra- 
violet : from these to the green the heliotropic effect gradually diminishes ; in the yellow it is zero : 
it recommences in the orange and gradually increases until it attains a second maximum (small) in 
the ultra-red. 
If the parts are not very sensitive, the effect is diminished in each of the colours in proportion 
to their heliotropic power, so that the orange, red, green, ultra-red, blue, etc. become inert in suc- 
cession. 
3. The heliotropic effect is not proportional to the mechanical intensity (thermic power) of 
the rays. 
4. Negatively heliotropic organs exhibit the same phenomena. 
III. Concomitant action of Heliotropism and Geotropism. 
In strongly heliotropic organs, geotropism does not interfere with the exhibition of heliotropism 
provided that the light is intense. 
IV. Presence of Oxygen. 
No heliotropic phenomena occur in the absence of oxygen. 
V. Photomechanical Induction. 
1 . Both heliotropism and geotropism are exhibited after the removal of the organ under experi- 
ment from the action of light or of gravity respectively : this effect is an induced effect. 
2. Successive exposures to the action of light or of gravity produce their effects distinctly ; there 
is no summation. 
VI. Relation of Heliotropism to Turgidity. 
In many cases positive heliotropic curvature does not take place in the zone of most rapid 
growth, where the turgidity is greatest, but in a zone below it, where the turgidity is less. 
Etiolated organs become more sensitive to the heliotropic action of light after they have been 
exposed on all sides to diffuse light, probably because the turgidity of the growing cells is thereby 
diminished.] 
