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POPULAR SCIENCE REVIEW. 
from bending* * * § , the cotyledons will accommodate themselves to the changed 
conditions by movements in a vertical plane. The cotyledon which points 
towards the light will sink, while the other will rise ; and thus both will 
become once more at right angles to the incident light. 
Two theories have been proposed to account for this property of leaves. 
The first is that of Frank,* who ascribes to leaves, and to some other organs, 
a specific sensitiveness to light, called ‘ Transversal-heliotropismus,’ or ‘ dia- 
heliotropism.’ t Just as an ordinary heliotropic organ has an inherent 
tendency to become parallel to incident light, so a diaheliotropic organ has an 
inherent tendency to place itself at right angles to the direction of the light. 
The two classes of organs differ from each other exactly as some creeping 
rhizomes do from ordinary stems ; the rhizome tends to extend itself horizon- 
tally under ground, while the stem above the surface grows vertically 
upwards. J 
A different theory has been proposed by De Vries, § whose views are 
supported by Sachs, || with additions or modifications. According to these 
views it is not necessary to assume the existence of any special kinds of 
heliotropism, since the phenomena might result from the ordinary forms of 
heliotropism and geotropism acting in concert. Thus, in the case of the 
seedling radish illuminated from above, if the cotyledons are apheliotropic 
(negatively heliotropic) and apogeotropic (negatively geotropic), it is possible 
that they might be kept in equilibrium by these opposing tendencies. The 
tendency to move away from a vertical light will make the cotyledons 
curve downwards toward the earth, and the apogeotropism, or tendency 
to move awav from the centre of the earth, may exactly balance the down- 
ward tendency, so that the cotyledons remain horizontal. Besides the 
various geotropic and heliotropic tendencies, there are other modes of growth 
which may enter into the combination. In some cases there is a natural 
preponderance of longitudinal tension and growth along the upper surface 
of the petiole, so that, owing to impulses arising within the plant, there is a 
tendency for the leaf to curve downwards;^! this tendency is called longi- 
tudinal epinasty, or simply epinasty : the opposite tendency is called hypo- 
nasty. 
According to the theories of De Vries and Sachs, epinasty may be 
opposed by heliotropism, or by apogeotropism and geotropism, and all these 
opposing forces may combine in producing an equilibrium. 
The object of the present paper is to test the relative values of the two 
above described theories, — that of Frank and that of De Vries and Sachs. 
The method employed was to fix the plants under observation to a hori- 
zontal spindle, which was kept in slow rotation by clockwork. This instru- 
ment (called the ‘ clinostat ’) has been employed by Sachs for the study of 
ordinary heliotropism ; light is admitted parallel to the axis of rotation, and 
* Die natiirliche wagerechte Richtung von PJlanzentheilen, 1870. 
t Darwin, Power of Movement in Plants ,’ p. 438. 
t See 1 Elfring,’ in Sachs’ Arbeiten, 1879. 
§ Sachs’ Arbeiten, i., 1872. || Sachs’ Arbeiten , ii., 1879. 
IT Or more accurately, in the direction in which the morphological lower 
side of the petiole is directed. 
