ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
591 
grasses, and Alga) ; and tlie observations were made by enveloping the 
organ in question in a thinner or thicker cylinder of gypsum. The 
power of resistance of growing plants increases in proportion to the 
resistance to be overcome, and varies so enormously that under certain 
conditions it may be as much as eighty-one time3 greater than in others. 
There is always, however, a limit to this power of resistance. Among 
Algae, Chara, Nitella , and Spirogyra retain their power of growth for 
months within an envelope of gypsum. The nodes of grasses are an 
especially favourable class of objects for determining the effects of geo- 
tropism ; since the geotropic tendency of the stem to bend upwards is 
displayed only in the nodes. In the growing tips of roots of Faba a 
longitudinal pressure of 5-19 atmospheres was measured; in roots of 
maize one of 9-21 atmospheres, in Vida sativa one of 8-13, and in the 
horse-chestnut one of 6 atmospheres. In roots of Faba the transverse 
pressure was from 2-6, and in maize 6 • 5 atmospheres. 
Mechanomorphosis.* * * § — Prof. J. Sachs maintains that natural selection 
can account only for the production of small variations, not for the 
differentiation of the main divisions of the natural system ; the isolation 
of the larger phylogenetic groups can by no means be explained by the 
law of the survival of the fittest. By the term mechanomorpJiosis he 
proposes to express the parallel changes in structure which are brought 
about in the different large groups by the action of similar external 
causes. Such is the production of leaf-like structures in Algse and in 
Phanerogams by the action of light on a tissue containing chlorophyll. 
Another example of mechanomorphosis is afforded by the action of the 
specific size of organisms on their internal structure and external form, j 
Mechanomorphosis may be of two kinds, barymorphosis and photo - 
morphosis, according as the principal agent in the changes is gravitation 
or light. The dorsiventrality of prothallia and the form of leaves are 
good examples of photomorphosis. 
Propagation of Potamogeton.j — M. C. Sauvageau records a series 
of observations' on the mode of growth, and multiplication of various 
species of Potamogeton, especially P. crispus, trichoides, pusillus, perfoli- 
atus, lucens , pectinatus, and natans , and on the anatomy of the stem. 
The ramification of all species of Potamogeton is of the same type : — 
the rhizome is always a sympode formed by the union of the first two 
internodes of successive generations ; in a few species the rhizome is 
wanting. In most species the fruit remains for a long time after maturity 
before germinating. All the species are readily propagated by fragments 
of the stem which become detached, and often hibernate before under- 
going further development. The portion which thus separates itself 
differs in the different species ; and usually presents some differentiation 
from the rest of the plant in its anatomical structure. In P. natans 
almost the whole plant hibernates in this way. 
Nutrition of Viviparous Mangrove-plants.§ — Herr G. Haberlandt 
describes a similar structure in other mangrove plants to that already 
* Flora, lxxviii. (1894) pp. 215-43. f Cf. this Journal, 1893, p. 659. 
+ Journ. de Bot. (Morot), viii. (1894) pp. 1-9, 21-43, 45-58, 98-106, 112-23, 
140-8, 165-72 (31 figs.). Of. this Journal, 1892, p. 63. 
§ Ann. Jard. Bot. Buitenzorg, xii. (1893) pp. 91-116 (3 pis.). Cf. this Journal, 
1890, p. 65. 
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