ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 591 



2. The diurnal sleep of leaves must not always be regarded as a 

 result of this action acquired to protect them from too great radiation ; 

 for if, in certain cases, either their position or the direction of the 

 rays of light is changed, they do not again place themselves in a 

 position to be illuminated by the most oblique incidence. 



3. The terms diaheliotropism and paraheliotropism, employed in 

 their wide signification, must therefore serve only to indicate the 

 positions of leaves in reference to the dii-ection of the rays of 

 light, without expressing an opinion on the causes which produce 

 them. 



Respiration of Leaves in Darkness.* — G. Bonnier and L. Mangin 

 find that, in the case of green leaves growing in darkness, the same 

 law of respiration prevails as in organs destitute of chlorophyll, viz. 

 that the relationship between the volume of carbon dioxide given off 

 and that of oxygen absorbed is constant, whatever the temperature, 

 both increasing rapidly with rise of temperature. 



Movements of the Sap in the Root-tubers of the Dahlia.! — 

 K, Kraus has confirmed the remarkable observation that a tissue with 

 acid sap may exude an alkaline fluid or at least one that becomes very 

 rapidly alkaline. The absorption of water by the tubers of the dahlia 

 takes place not at all or very slightly through the surface, but 

 almost entirely through the roots which are produced in abundance 

 in October and November. An abundant " bleeding " or exudation 

 of sap takes place from uninjured leaves and from the axils of the 

 leaves ; also from transverse sections, which ceases as soon as the 

 tubers are deprived of their roots. 



There is not in the tubers any sharj) distinction between the 

 medullary rays and the xylem-parenchyma, the mass of the root con- 

 sisting mainly of rows of radially elongated parenchymatous cells. 

 There are, however, darker portions composed of tracheids surrounded 

 by a sheath of thick-walled parenchyma, corresponding to the " fibre- 

 cells " of normal wood. When the tubers are cut off sap exudes 

 especially from the periphery of the xylem, most abundantly on 

 transverse and tangential sections, and proceeding mostly from the 

 closely inclosing parenchyma, partially also from the sieve-tubes and 

 pith. That which exudes from the sieve-tubes is alkaline, from the 

 wood and pith acid ; inulin was also detected in it. The cause of 

 this exudation of sap the author regards to be tension of the tissues. 

 After the alkaline " bleeding " from wounded tubers has ceased, the 

 formation of cork cf>mmenccs on the wounded surfaces. The alkaline 

 reaction is probably the result of decomposition. 



Absorption of Water by the Capitulum of Compositae.j — A. Bur- 

 gerstcin notices that the flowers of Composita! possess the faculty of 

 absorbing water from without through the epidermis ; and that the 

 under side absorbs water more rapidly than the upper side. 



• CompleH RendnB, xcviii. (1884) pp. 1064-7. 



t Wollny'H Forwh. au8 dem Geb. der AgriciiUui-{)liysik, vi. (1884) 8ee 

 IJot. CVTilrall.l., xviii. (1884) p. 65. 



J Ber. DeutHch. Dot. Gcsdl., i. (1883) pp. 367-70. 



