145 
VI. The Iufluence of Light upon the Growth of unicellular Organs. 
called heliotropism ol the plasmodia of Myxomycetes and the change of 
Position exhibiled by the chlorophyll-grains of different Organs when ex- 
posed alternalely to light and lo darkness prove, that light can act di¬ 
rectly upon protoplasm. Pfeffer') recently concluded upon purely phy- 
sical grouuds, that the diminution of extensibility which cells undergo 
when exposed to light, was due lo the tnolecular aclivity of light in the 
primordial utncle (plasma-membrane). 
If this be the correct explanation of the action of light in retarding 
grovuh, it must be applicable also to the phenomena of heliotropism and 
gans 1<>Se leSUllin§ fl0ni the action °f light upon spontaneously motile or- 
ln considering those special cases of the action of light upon growth, 
W are inc, uded under the term »heliotropism«, I shall only allude 
lo those in which the heliotropism is positive, for the true nature and 
signihcnnce of negative heliotropism is at present quite uncertain. The 
cuivature is due, as Sachs has shewn, to a greater rapidity of the growth 
ol the side most distaut from the source of light. This is stated of a 
multicellular structure, but there is no reason to believe that the case is 
hei v\ ise with a single cell. The protoplasm of the concave side may 
have had its extensibility diminished by the action of light and conse- 
quently u s growth is less rapid than is that of the convex side. Pfeffer 2) 
ascribes the heliotropism of unicellular organs to an action of light upon 
the cell-wall. This Suggestion is, however, not in harmony with his 
View already quoted, that light can directly alfect the extensibility of 
the plasma-membrane. Furlher it compels the assumption that there‘are 
two kinds of positive heliotropism, viz, that of multicellular organs due 
to a gradual diminution of the turgidily of the cells extending from the 
less stro-ngly to the more strongly illuminaled side, and that of unicellu¬ 
lar organs. Under these circumslances it is evident, that the explanation 
given above offers fewer difliculties. 
With reference to the sponlaneous movements, it is well-known, that 
they are generally arrested when the plants, which exhibit them are ex¬ 
posed to light. Sachs 3) has already drawn attention to the similarity 
between this phenomenon and the retardation of growth effected by light, 
and has suggested that the mode of action is the sarne in both cases. 
The mechanism of the, sponlaneous movements has not been directly 
invesligatcd, but they are so similar to the movements which are indu- 
ced by Stimulation that our knowledge of the mechanism of the laller 
may be fairly applied to explain that of the formet - . The mechanism of 
U Bot. Zeitg. 1876. p. 77. 
2) Osmot. Untersuch. 1877. p. 208 . 
3) Lehrbuch p. 852. 
Arbeiten a. d. bot. Institut in Wurzburg. Bd. II. 
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