Diatropism. 123 
their normal position, they return to it more rapidly than when they 
are below the normal position and have to bend upwards. This has 
been supposed to mean that in their downward curvature they are 
helped by pros-geotropism, whereas their upward bend is made 
against such influence. I prefer to look at the upward and down¬ 
ward curvatures as comparable to the flexion and extension of a 
man’s arm, and according to this view there seems no reason why 
one movement should not be stronger than the other. 
Before leaving the subject of lateral roots it should be pointed 
out that although their line of growth is determined by geo¬ 
perception, yet in their earliest stage of development the angle that 
they make with the main root is to a large extent determined by 
internal stimuli. Thus in a root which has grown on the klinostat 
the developing secondaries make nearly the same angle with the 
main root that they would were the plant not on the klinostat, but 
growing normally. It is, however, said that in the klinostat 
specimen the secondary roots make a slightly greater angle with 
the primary root than is usual. 
Other diageotropic organs are the underground runners of 
Scirpus, Eleochavis and Spavganium. Here Elfving 1 observed the 
interesting fact that if the aerial parts of the plant are injured, the 
stolons become apo- instead of diageotropic. We have in fact a 
change in reaction similar to that produced in lateral roots by the 
amputation of the primary root. 
Among aerial organs certain leaves are clearly diageotropic, 
but, perhaps, the most interesting problems are given by pine 
branches which continue to grow horizontally in darkness, and are 
undoubtedly diageotropic. Frank 2 found that when a pine branch is 
bent up and fixed so that the apex points vertically upwards, it regains 
the horizontal (in some cases) by bending so that the lower side of 
the branch is concave. In other cases the upper side of the branch 
is concave, and here the branch is necessarily upside down in the 
horizontal position: this however is corrected by torsion. In the 
same way branches of Conifers, fixed horizontally with the upper 
side downwards, undergo torsion. If this experiment is made 
before the terminal buds develope, no torsion occurs in the branch, 
but by appropriate twisting of the leaves the branch acquires an 
approximately normal appearance. 
The flowers of Narcissus 3 afford a good instance of dia- 
1 Sachs’ Arbeiten ii. 
2 Die natiirliche wagerechte Richtung von Pflanzentheilen. 
Leipzig, 1870. 
3 Vochting, Die Bewegungen d. Bliithen u. Friichte, 1882. 
