550 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
dry summer of 1893. The result was a general retardation of growth, 
especially in the spots most exposed to the sun ; this retardation was 
exhibited in a diminution, both of the diameter of the trunk and of the 
length of the shoots ; the breadth of the woody layer formed during that 
summer was not above from two-thirds to three-fourths of the average. 
As a general rule the upper part of the trunk appeared to be the portion 
least affected, this being the region where the starch produced in the 
leaves is first consumed. 
Ascent of Sap.'*' — Herr E. Askenasy comes to the same general con- 
clusions as Dixon and Joly with regard to the cause of the ascent of sap 
in trees, attributing it to the effect of transpiration in the leaves. The 
drawing force is carried from above downwards by the cohesion of the 
particles of water in the conducting paths, and by the adhesion of the 
water to the walls of the vessels. 
(3) Irritability. 
Irritability of Water-plants.f — Prof. M. Mobius has investigated 
the sensitiveness to light of some aquatic plants, especially of Cerato - 
phyllum demersum. If shoots which swim erect in the water remain in 
the dark for some days, the internodes lengthen considerably, and 
the leaves, which are normally directed upwards, droop, as also do the 
lateral branches. The elongation does not take place, as is the case 
under similar circumstances with land-plants, chiefly in the apical region 
beneath the terminal bud, but mainly in the older internodes ; and this 
is effected by a lengthening of the cells in that region. Further observa- 
tions showed that these movements are not geotropic or chemotropic, but 
entirely heliotropic. The plant exhibits a moderate positive helio- 
tropism ; but, in the absence of light, peculiar movements take place 
which originate in the primary axis. Darkness appears to exercise a 
directly irritating effect on the plant. 
Very similar results were obtained with Myriophijllum ; and a similar 
elongation of the cells of the internodes in darkness takes place in 
Elodea canadensis. In other water-plants — Ranunculus, Naias, Cabomba 
— no such phenomena were observed. The author compares these 
results with earlier observations on Gallitriche and Chara. In Elodea 
canadensis darkness has a remarkable effect in promoting the formation 
of roots. 
The phenomena above described differ in some respects from all 
others hitherto observed in the vegetable kingdom, and deserve further 
investigation. The author speaks of it as a kind of position-paralysis 
( Lagestarre ). 
Heliotropism. f — Herr J. Wiesner replies to several points in 
Eothert’s recent publications on this subject, and points out that his 
own observations and conclusions have, in several instances, been 
seriously misrepresented by that writer. 
* Yerhandl. Naturh.-med. Yer. Heidelberg, v. (1895) 23 pp. See Bot. Centralbl. 
lxii. (1895) p. 237. Cf. this Journal, ante, p. 335. 
t Biol. Centralbl., xv. (1895) pp. 1-14, 33-44 (8 figs.). 
X Bot. Ztg., lii. (1895) 2 te Abt., pp. 1-13. Cf. this Journal, ante, pp. 73, 75. 
