42 
Francis Darwin. 
Movement (p. 468, &c.) consisted in checking the heliotropism of 
Phalaris, A vena, &c. by covering the tips of the seedlings with 
opaque caps. The experiments were thought to prove that light- 
perception is confined to the apices of the seedlings in question. 
But Rothert 1 has proved that this is not accurate: it is true that in 
Phalaris (for instance) the tip is by far the most sensitive region 
and finally determines the curvature of the plant, but the basal 
part of the cotyledon is also sensitive. As is well known Rothert 
further showed that in certain genera of grasses, e.g., Setaria, 
Sorghum, and Panicum, there is a sharply marked localisation of 
perception, which is confined to the cotyledon, while the movement 
is chiefly, but not exclusively, confined to the hypocotyl. A 
question of terminology is illustrated by the plants in question, 
Rothert points out that empfindlich (perceptive) and reizbar 
(irritable) are applicable to distinct regions of a Setaria seedling. 
The cotyledon is photo-perceptive, the hypocotyl not so : but the 
hypocotyl is irritable since it is put in motion by an influence 
coming from the region of Empfindlichkeit. 
In dicotyledonous seedlings the most sensitive region is at the 
summit of the hypocotyl just below the cotyledons, while the base of 
the hypocotyl may also be sensitive. Rothert has shown (ibid, p. 129) 
that this state of things is not confined to seedlings but occurs in 
the shoots of Dicotyledons. He worked with young Dahlias grown 
from tubers, and arranged his experiment so that a given plant was 
exposed to lateral illumination “ capped ” and “ uncapped ” on 
alternate days. The average amount of curvature from the 
vertical in th» capped specimens, i.e., those whose apices were 
covered with an opaque tube, was 29°, in the uncapped plants the 
curvature was 55°. A single shoot, which behaved with great 
regularity gave the following angles with the vertical on alternate 
days, the underlined figures being the results obtained when the 
tips of the plants were covered, 
75°, 50^, 65°, _20_°, 45°, 5_°, 
1 shall briefly return to the localisation of perception in 
discussing dia-heliotropism. 
1 Cohn’s Beitriige VII. 
