74 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
was observed in all the organs examined, and even, in the case of seed- 
lings of Panicese, from the cotyledon to the hypocotyl. It takes place 
invariably in the basipetal direction, and usually but slowly. The irri- 
tation is transmitted through the fundamental parenchyme, the vascular 
bundles taking no essential part in it. As a rule (in opposition to 
Darwin’s view), the entire region over which the curvature extends is 
heliotropic, but in the seedlings of certain PaniceaB the cotyledon only is 
sensitive, the hypocotyl curving only under the influence of an irritation 
conveyed to it from the cotyledon. The degree of sensitiveness, how- 
ever, varies greatly in different parts of the organ. In all the cases 
hitherto accurately investigated, it is only a very short apical region in 
which heliotropic sensiveness is very strongly marked ; in the whole of 
the rest it is usually very much weaker. 
The author distinguishes between lieliotropic sensitiveness ( Emp - 
findlichkeit) and irritability ( [Deizbarkeit ), which are dependent on two 
different properties of the protoplasm. Irritation ( Reizung ) may be 
direct or indirect, i. e. may be the result of a local sensitiveness, or can 
be transmitted. The power of lieliotropic curvature is dependent on four 
factors, — the anatomical structure, the thickness, the intensity of growth, 
and the heliotropic irritability of the organ or of the portion of an organ. 
It is not necessarily dependent on the sensitiveness of the part ; the 
curvature may be the result of an irritation transmitted from another 
part. For the same reason the maximum power of curvature does not 
necessarily coincide with the maximum intensity of growth. 
So far as curvature is the result of growth, the power of heliotropic 
curvature ceases with the cessation of growth. Heliotropic sensitiveness 
and irritability are, on the other hand, entirely independent of growth, 
and continue after the power of growth and of curvature have completely 
ceased. There is no such thing as the “ traction-growth ” of Wiesner. 
The decapitation of the seedling of grasses has two results ; a greater 
or less diminution of the intensity of growth, and an entire destruction 
of its heliotropic and geotropic sensitiveness ; but both effects are only 
temporary. They are not the result of the removal of the apex, but of 
the cut, which causes the transmission of an irritation in the basipetal 
direction. 
Movements of Zygomorphic Flowers.* — From observations of 
zygomorphic flowers, chiefly of Aconitum and Delphinium, in nature, and 
also from experiments with the clinostat, Dr. B. Meissner supports the 
theory of Noll, rather than that of Schwendener and Krabbe, with regard 
to the causes of their characteristic movements of position ( Orieniirungs - 
bewegungen). He finds no evidence of the action of the force called by 
the latter writers geotortism. The lateral movements of the flower-stalk 
are, on the other hand, the result of the action of gravitation. 
Geotropic Sensitiveness of the Root-tip.f — Prof. W. Pfeffer de- 
scribes a series of experiments by which he proves that in uninjured 
roots it is the tip only that is geotropically sensitive. Boots ( Faba , 
Lupinus , &c.) were made to grow into short tubes of thin glass bent at a 
right angle. By this means a short terminal portion 1*5-2 mm. long 
was forced to grow at right angles to the rest of the root. To prevent 
* Bot. Centralbl., lx. (1894) pp. 1-15 (8 figs.). Cf. this Journal, 1893, p. 354. 
t Ann. Bot., viii. (1894) pp. 317-20. 
