ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
505 
grown under conditions where the mode of growth of the roots could bo 
clearly followed out, showing the closest resemblance to that of the roots 
of epiphytes. 
Changes of Pressure in Mimosa.* — M. G. Bonnier has studied, by 
means of a manometer, the changes of pressure which take place in the 
motor cushion of the leaf of the sensitive plant when the leaflets are 
displaying their sensitive movements. He finds that, when the leaves 
are irritated, the pressure at the base of the lower surface of the motor 
cushion diminishes, and that it again increases as the leaves assume 
their normal position. If the leaves are maintained in this position by 
the action of chloroform, the pressure in the motor cushion remains 
unchanged. A decrease of pressure in the surrounding air causes a 
movement in the reverse direction to the sleep position, but does not 
communicate itself to the tissues of the plant. The variations in the 
movements of the leaflets are always connected with variations in 
the internal pressure of the motor cushion. 
(4) Chemical Changes (including Respiration and Fermentation). 
Physiology of Succulent Plants.f — M. E. Aubert has undertaken 
an exhaustive series of observations on various points connected with 
succulent plants. Those examined belonged chiefly to the Cactaceae, 
Mesembryanthemaceae, and Crassulacese ; but the degree of succulence 
in the various species of these orders varies greatly. 
With regard to their assimilation and respiration, he finds that the 
atmosphere contained within the tissues of these plants differs from 
the surrounding air in the proportion of the gases of which it is com- 
CO 
posed. The value of the fraction — - 2 varies in direct proportion to the 
succulence of the species. The Crassulaceae and Mesembryanthemaceae 
with a thin cuticle manifest a greater exchange of gases with the sur- 
rounding atmosphere than most of the Cactaceae, while the succulent 
Euphorbiaceae are intermediate between the two. The two processes of 
assimilation and respiration proceed simultaneously in succulent plants, 
and the value of the fraction — is in direct proportion to their succulence. 
With regard to the organic acid contained in the sap, which has so 
great an influence on the turgor of the cells, M. Aubert finds that in 
the Crassulaceae it is malic (isomalic) acid, free or combined, with 
traces of tartaric ; in the Mesembryanthemaceae it is oxalic acid ; in the 
Cactaceae it is malic acid associated with a large quantity of gums. The 
amount of malic acid in the leaves of the Crassulaceae increases from 
the terminal bud to a point in the stem where the leaves have almost 
attained their maximum development; from this point it decreases in 
the adult leaves, but never completely disappears. The amount of malic 
acid also varies in different parts of the same leaf; it is least in those 
* Rev. Gen. de Bot. (Bonnier), iv. (1893) pp. 513-28 (2 pis.) ; and Bull. Soc. Bot. 
France, xxxix. (1893) pp. 365-8. 
f Ann. Sci. Nat., xvi. (1892) pp. 1-90 (9 figs.); and Rev. Gen. de Bot. (Bonnier), 
iv. (1892) pp. 203-19, 273-82, 320-31, 337-53, 373-21, 421-41, 497-502, 558-68 
(1 pi. and 5 figs.). 
