ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 549 



nearly the importance of tbe ascending current, it is not without physio- 

 logical signiticance to the plant. The author states that it has a 

 detiuite influence in the opening of many flowers and inflorescences, in 

 consequence of the passage of water from the flowers themselves into 

 the foliage beneath them, in the formation of sympodial leafy shoots, 

 terminal and axillary buds, &c. 



Influence of Light on the Development of Bark.* — M. H. Douliot 

 finds that the develoj)raeut of bark does not depend in any way on the 

 action of gravitation, the under and upper sides of a horizontal branch 

 being quite homogeneous when equally illuminated. But on an erect 

 stem the development of bark is always greater on the south side, which 

 receives a larger amount of light, than on the north side. The author 

 suggests that this may be due to greater transpiration, moisture acting 

 imfavourably on the formation of periderm. 



Periodical Activity of the Cambium in the Roots of Trees.f— By 

 observations matle on seventeen species of trees belonging to the 

 Dicotyledons and Conifers, Herr L. A. Gulbe has determined that the 

 activity of the cambium begins in the spring in the slender branches, 

 advances from there to the trunk, then into the thicker, and finally into 

 the slender roots, about four or five weeks from the commencement of 

 the activity. In the autumn the decrease of activity takes place in the 

 same period, varying within about two months, and finally ceasing in 

 the second half of October, 



Penetration and Escape of Gases in Plants. J — M. L. Mangin gives 

 the following summary of the law^s which regulate the penetration and 

 exhalation of gases in plants. 



The diffusion through cutinized surfaces is independent of variations 

 in temperature within the limits of the plant; it is, fur each gas, pro- 

 portional to the difterence of the pressures which the gas exercises on 

 the two surfaces of the membrane. The rapidity of diffusion varies with 

 different gases. The coeflicient of permeability, i. e. the amount of 

 carbon dioxide diffused per hour and per square cm. of surface, is 

 greater in the case of submerged than of aerial leaves ; when the two 

 surfaces of the leaf are unlike, it is usually greater on the lower than on 

 the upper surface. The permeability does not depend on the thickness 

 of the cuticle, but altogether on the extent to which it is impregnated 

 with wax, a certain amount of waxy matter being present on all leave? 

 whether submerged or aerial. The duration of tbe life of leaves influ- 

 ences their permeability, deciduous being often more permeable than 

 persistent leaves ; other things being equal, the number or size of the 

 stomates increases in j)roportion as the permeability decreases. The 

 closing of the stomates by a covering which preserves intact the permea- 

 bility of the membranes diminishes the exchange of respiratory gases 

 from one-fifth to one-half; but the diminution ceases when the tem- 

 perature is low ; it is greatest with young and deciduous leaves. The 

 diminution of respiration on the closing of the stomates is due entirely 

 to the insufficient supply of oxygen. The process of assimilation is also 



* Journ. de Bot. (Morot), iii. (1S89) pp. 121-4 (8 figs.). 



t Arb. St. Petersburg. Naturf. Gesell., xviii. p. 45. °See Bot. Ctntralbl., xxsviii. 

 (18S9) p. 487. 



J Ann. Sci. Agrm. Frang.et Etrang., i. (1888). See Joiu-n. de Cot. (Morot), iii. 

 (1889) Kev. Bibl., p. iv. Cf. this .Journal, 1888, p. 763. 



