54 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
investigated the cause of this by growing plants in a pot strongly illu- 
minated from one side. The hypocotyl is orthotropous, and is positively 
phototropic when illuminated on one side. This property is retained 
during the whole of its development ; it is never plagiotropic, and never 
becomes phototropic even on the strongest insolation. The experiments 
made determined that light is the cause of the plagiotropic change in 
the originally orthotropous shoot. 
The origin was also investigated of the inversion of the lower part 
of the lamina displayed by many (but not all) species of Alstroemeria , 
a phenomenon exhibited also by a considerable number of grasses, and 
by some other plants. The examination of very young leaves of A. 
j psittacina and pelegrina showed that the stomates are developed on 
the morphological upper surface before the torsion becomes externally 
visible on the erect unopened young leaves. Experiments with the 
clinostat showed that the cause of the torsion is photo-irritation. The 
author advocates the view that, in the course of the phylogenetic develop- 
ment of the genus, the twisted have originated from erect paraphoto- 
tropic leaves ; this view being confirmed by the fact that there are still 
some species with erect leaves. 
Contact Irritability in Hook-climbers.* — According to Dr. A. J. 
Ewart, there is every form of gradation between the simpler form of 
climbing hooks, such as occur in Uncaria , and the highly specialised 
tendrils of the Passiflora type. From the simplest type (Bubus fruti - 
cosus, Acacia, Csesaljpinia , &c.) where the hooks are simply protective 
and their use in climbing purely accidental, we pass to others ( Luvunga ) 
which possess both non-irritable spines and irritable climbing hooks, 
and a further stage ( TJncaria, Artabotrys ) in which the spines have dis- 
appeared, and the irritable clasping hooks alone remain. A further 
advance is shown by Boucheria and Ancistrocladus, where, besides the 
thickening, the sensitive region shows a slight increased curvature due 
to contact alone. The differentiation between concave and convex sur- 
faces, as regards irritability, is more strongly displayed in Bauhinia. 
The highest type is represented by the tendrils of Cucurbita, Passiflora, 
Sicyos, &c., and the root-tendrils of Vanilla, which exhibit rapid curva- 
ture as the result of contact alone. 
No hard and fast line can be drawn between a contact and a pressure 
stimulus. Irritable hooks respond to pressure more readily than tendrils, 
the irritability being confined almost entirely to the concave surface of 
the hook. When the curvature is produced slowly, it is probably due 
to an unequal growth of the cambium of the two surfaces. 
Heliotropism caused by Diffused Daylight.f — Herr J. Wiesnor has 
taken a series of photographs of the seedlings of a dicotyledon ( Vida 
sativa ) grown in diffused daylight. The following are the chief con- 
clusions. 
Although the organs of a plant often possess an enormous capacity 
for heliotropic reaction, they always grow towards the strongest light 
when illuminated by diffused light which radiates from all sides. The 
part which has become heliotropic divides the area of light which 
* Ann. Jard. Bot. Buitenzorg, xv. (1898) pp. 187-242 (2 pis., 5 figs.) (English). 
f Ber. Deutsch. Bot. Gesell., xvi. (1898) pp. 158-63. 
