358 
SUMMARY OP CURRENT RESEARCHES RELATING TO 
without effect, a sharp w r ater impact or immersion so as to cover one or 
more bristles, causes closure. The irritability is not confined to the 
bristles, but belongs, in a modified extent, to the whole surface of the 
leaf. A gradual increase in temperature promotes stimulation, but a 
strong light has no obvious effect. 
A full description is given of the secreting glands on the surface of 
the leaf, as well as of the sensitive bristles. The bristle is not a true 
hair, but an emergence, and consists of three well-marked regions, the 
base, the joint, and the shaft. The central portion or joint is the part 
which is specially irritable. The epidermal cells exhibit abundant 
continuity of protoplasm. The bristles communicate, by their epidermal 
and mesophyll portions, with the epiderm and mesophyll of the lamina 
of the leaf, w r hile the cells of the mesophyll of the lamina appear to be 
cut off from those of the epiderm by a thickened wall, but communicate 
directly with the gland-cells. The secretion of the gland-cells is 
entirely due to irritation of the protoplasm, and is poured out alike as 
the result of mechanical, chemical, and electrical stimulus. Previous to 
secretion the leaf is in a state of tetanic contraction. 
The author points out the remarkable analogy between the pheno- 
mena of irritability in the leaves of Dionsea and those of the contracti- 
lity of animal muscle. 
Movements of the Leaves of Melilotus.* — Dr. W. P. Wilson and 
Mr. Jesse M. Greenman point out that, in addition to the normal day- 
light position and the night position, the leaves of Melilotus alba , and of 
many other plants belonging to the Leguminosse and to other natural 
orders, have a third or hot sun position. This position is not dependent 
on light alone, but also on the heat rays ; and its object is to protect 
the plant from a too rapid transpiration, by exposing as small an amount 
of surface as possible to the direct rays of the sun. This is effected in 
very different ways in different plants. 
Heterogenous Induction.! — Herr F. Noll discusses the relationship 
between heliotropism and geotropism. The nyctitropic movements of 
leaves may very well, he considers, be of a purely geotropic nature, 
dependent on the different anatomical structure of the two sides of the 
leaf. Those sensitive movements in which two different causes co- 
operate in producing the final result he terms “ heterogenous induction,” 
in contrast to “isogenous induction,” resulting from one only of the 
two causes. Those organs he calls “ homalotropous ” which grow in a 
horizontal direction. Coiling is a geotropic phenomenon. The seat of 
the sensitive phenomena be believes to be the parietal utricle of the 
protoplasm. Heterogenous induction is a widely distributed pheno- 
menon. The ordinary horizontal position of leaves is, for example, the 
result of the combined action of geotropism and heliotropism. 
Cause of Physiological Action at a Distance.! — M. L. Errera 
attributes the action of pieces of iron in attracting the growing sporan- 
* Contrib. from the Bot. Laboratory of the Univ. of Pennsylvania, i. (1892) 
pp. 66-73 (5 pis.). 
t ‘ Ueb. heterogene Induction u.s.w.,’ Leipzig, 1892, 60 pp. See Bot. Centralbl. 
liii. (1893) p. 287. \ Ann. Bot., vi. (1892) pp. 373-5. 
