454 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
screens the leaflets remain open as in ordinary daylight. In all cases 
nyctitropic movements are accelerated under a red screen ; not quite 
so strongly under a yellow screen ; under a green screen the move- 
ments practically coincide in time with those of exposed plants, and are 
beautifully regular in sequence ; under blue light there is a distinct 
retardation of the normal nyctitropic period. Up to 38°, or even 43° C. 
in some species, heat-rays appear to fail in stimulating the tissues. The 
general result is that the heat-rays, the less refrangible light-rays, and 
the more refrangible light-rays, are all trophic up to a certain point. 
Orange, yellow, and green screens to the protoplasm, whether in the 
form of pigmented walls, pigmented cell-sap, or chlorophyll, are of a 
protective character, and permit the normal functions to be carried on 
unimpeded by the injurious action of the more intense blue- violet rays. 
Geotropism.* — From a series of observations on this subject made 
by Herr F. Czapek, he concludes that, as a rule, the seat of the geo- 
tropic sensitiveness of the root lies in its apex, over a zone of from 
1 • 5 to 2 mm. immediately below the root-cap. The geotropic curvature 
is, however, chiefly manifested in a zone at some distance from the 
sensitive portion. In older portions of the growing stem, on the other 
hand, the sensitive zone coincides for the most part with that of growth 
and curvature. The conditions which favour geotropic curvature are 
the same as those which favour growth in length ; while curvature and 
sensitiveness are dependent on different conditions. The maximum of 
geotropic curvature is reached when the organ deviates from the hori- 
zontal at an angle of about 45°. This condition is attained when the 
root of a seedling grows obliquely upwards or a shoot obliquely down- 
wards. In secondary roots the maximum geotropic curvature appears to 
occur at an angle of from 60° to 90°. 
Geotropic Curvature of Nodes.f — Herr R. Barth has investigated 
this subject in a variety of plants. He finds the position of the motile 
zone to vary. It may be situated at the base of the internode, or, less 
often, at its apex, or both base and apex may take part in its formation. 
In some cases it is only the portion of the stem which is enclosed in the 
base of the leaf that curves ; in others only the portion of the node 
which belongs to the leaf-sheath ; in others again both stem and sheath 
take part in the geotropic curvature. Motile nodes are in general dis- 
tinguished from the rest of the stem by the absence of sclerenchyme and 
hard-bast, while the collenchyme is usually very strongly developed, 
although it may be replaced by thin-walled parenchyme. In some 
plants geotropic curvatures are manifested only in the nodes, while in 
others the whole of the growing internode takes part in the upward cur- 
vature. In those leaves in which the leaf-sheath is strongly developed, 
the movements take place in some only so long as growth continues, the 
leaf-sheath being usually passive ; while in other instances the geotropic 
reactions continue in the fully developed node. In many grasses the 
leaf-sheath as well as the stem participates in the movements. The 
* Jahrb. f. wiss. Bot. (Pfeffer u. Strasburger), xxvii. (1895) pp. 243-339 (1 pi.). 
Cf. this Journal, ante , p. 74. 
t 1 Die geotropischen Wachsthuraskriimmungen d. Knoten,’ Leipzig, 1894, 39 pp. 
See Bot. Centrulbl.. lxi. (1895) p. 364. 
