Diatropism. 125 
upwards, until both are at right angles to the line of light. It is 
necessary to prevent the hypocotyl curving towards the light, 
otherwise the plant can adapt itself to oblique illumination by 
simple heliotropism of the stem. If a similar experiment is made 
with a plant whose leaves are decussate it will be evident that the 
leaves which do not point to or from the source of light cannot 
acquire the light-position by simple curvature. They can, however, 
place themselves at right angles to the oblique light, but this is 
effected by torsion, not by flexion. Frank 1 was the first to give 
evidence that the capacities just decribed are strictly analogous to 
positive or negative heliotropism, differing merely in the form of 
the reaction. This view was opposed by De Vries who, in an 
elaborate research 2 made in Sachs’ laboratory, tried to show that 
diaheliotropism is not a quality sui generis but is the outcome of a 
balance struck between such tendencies as apogeotropism, aphelio- 
tropism, epinasty, hyponasty, and the result of the weight of the 
leaf. In criticising De Vries’ theory the first question that 
arises is :—are these leaves apheliotropic or apogeotropic ? I shall 
show later that it is impossible to allow that they are either 
positively or negatively heliotropic. With regard to apogeotropism 
it is usual to assume because the leaves of certain plants rise up 
vertically when placed in the dark, that this is due to apogeotropism 
which existed when the plant was in the open air but was checked 
by apheliotropism. But this is not a logical conclusion, it is 
conceivable that we have here an effect similar to the downward 
bend of lateral roots produced by illumination, and that the 
phenomenon must be classed with other changes in “ Stimmung ” 
produced by external conditions. 
Some years ago I was able to prove 3 that at least some leaves 
can regulate their position as regards light without any assistance 
from geotropism. The method consisted in keeping plants slowly 
rotating on the horizontal axis of a klinostat, and at the same time 
subjecting them to one-sided illumination. If the theory of 
De Vries is correct plants so treated ought not to be able to keep 
their leaves at right angles to the light, since gravitation, one of 
the elements in the supposed balance of stimuli, is wanting. I 
found however that at least some plants are capable of finding and 
retaining the light-position on the klinostat. 
1 Bcitriige zur Pflanzeiiphysiologie, 1868. 
1 Sach’s Arbeiten i. 
1 Journal Linn. Soc., 1881. 
