ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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themselves against injury from long-continued rain or other exposure 
to moisture. Such plants belong mostly to the xerophilous vegetation 
of temperate climates. They may be arranged under four types, viz. : — 
(1) Plants whose flowers close their perianth in rainy weather in such 
a way that drops of rain cannot enter (or only with difficulty) the 
flowers which are open in fine weather ; the position of the flowers or 
inflorescence not being altered. (2) Plants in which, after the flower 
opens, the flower-stalk curves in rainy weather, in such a way that the 
opening of the flower, previously directed upwards, changes its position 
so as to protect the pollen or nectar against the rain. (3) Plants in 
which a similar movement takes place in the stalk of the inflorescence. 
(4) Plants in which the movements characteristic of the second and 
third type are accompanied by a closing of the perianth. In addition, 
the author gives a list of flowers in which the protection of the pollen 
against rain does not depend upon a photo-dynamic principle. 
Curvature of Roots.* — Prof. D. T. Macdougal has carried out a 
series of observations on the localisation of the curvature of roots, and 
the causes which give rise to it. Curvatures in general he regards as 
due to changes in the cell-wall rather than in the osmotic activity of 
the cell-contents. But the curvatures of stems are not identical with 
those of most tendrils or of many roots. The curvature of roots is 
due t to the excessive active elongation of the internal layers of the 
cortex of the side becoming convex, made feasible by the increased 
stretching capacity of the longitudinal membranes. In consequence of 
the stretching, the membranes of the convex side become thinner ; and, 
as a later effect, the membranes of the concave side become thicker. 
The organs of the irritable mechanism of roots exhibit a physiological 
rather than a morphological differentiation. The part of the root 
affected may be divided into a sensory zone and a motor zone. The 
movement of a root is caused by changes in the region in which the 
energy of the periblem is turned from cell-division to cell-enlargement. 
The sensory zone consists of a cup-shaped mass of periblem extending 
1-2 mm. axially, from which the bottom, representing the growing point, 
is wanting. The sensory zone extends approximately to the forward 
edge of the motor zone. The motor zone includes a length of 2-3 mm. 
The curvatures of roots apical and basal to the motor zone are me- 
chanical accompaniments of the action of the motor zone. 
Rheotropism and Thermotropism in a Plasmode.j — Mr. J. B. 
Clifford has studied these phenomena in the sclerote of a Myxomycete, 
probably Fuligo varians , which could be induced to assume the active 
plasmode condition by supplying it with abundant moisture. Up to a 
certain point the plasmode was found to be positively rheotropic, but 
a very slight increase in the strength of the current of water caused it to 
become negatively rheotropic ; any considerable increase in the strength 
of the current induced it to move entirely away from the water. As to 
thermotropism, the plasmode will live in an atmosphere ranging from 
— 2° to 52°-53° C. ; it remains positively thermotropic up to 33°-34° C., 
becoming negatively thermotropic above that point. 
* Bot. Gazette, xxiii. (1897) pp. 307-66 (1 pi. and 7 figs.). 
t Ann. of Bot., xi. (1897) pp. 179-86 (3 figs.). 
