ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 827 



co-operation of a large numbor of small causes, but chiefly to un- 

 equal growth, caused by the attractive force exercised by the more 

 refrangible rays of light on the chlorophyll-bodies, in consequence of 

 which the cells on the side of the stem exposed to the sun are often 

 larger than those on the shaded side. The same applies also to the 

 twining of stems; and the author believes that this may afford an 

 explanation of the apparently anomalous fact that light exercises a 

 retarding influence on growth. 



Causes of Twining.* — Dr. J. Wortmann sums up the facts known 

 relating to the cause of the twining of stems, and concludes that it is 

 due to a movement of growth only distinguishable from those of 

 ordinary orthotropous stems which grow in a vertically ascending 

 direction, in being caused, not only by negative geotropism, but also by 

 rotating nutation which acts in a horizontal direction. Tho combina- 

 tion of these causes is sufficient to account not only for all true 

 twining movements, but also for the torsions of the stem round its 

 own axis, whether homodromous or antidromous, which result as 

 secondary phenomena. A negatively geotropic shoot which is growing 

 with sufficient rapidity may in this way acquire the faculty of coiling 

 round a support. This view is supported by the observation of Noll, - ] - 

 which Wortmann confirms, that an etiolated seedling can, through 

 the operation of circumnutation, coil like a true climbing plant. 



Metastasis in the Crassulaceae.J — Dr. G. Kraus .has made a 

 detailed examination of the composition of the cell-sap in the succulent 

 Sedums, Sempervivums, and other Crassulacea), and of tho chemical 

 changes which go on in it. 



In addition to free malic acid he finds a very large amount of 

 calcium malate ; analysis of the leaf-rosettes of Sempervivum showed, 

 in the dry weight, 3 ■ 2 per cent, free malic acid, 25 • 9 per cent, calcium 

 malate, \' 5 per cent, sugar, 7*2 per cent, starch. The calcium 

 malate acts the part of a reserve food-material, accumulating during 

 the period of active growth for use when the flowers and fruits are 

 being formed. For this purpose it is probably first decomposed into 

 carbohydrates. The free malic acid is formed chiefly in the night, 

 and is therefore not a direct product of assimilation, but is apparently 

 the result of the activity of assimilation during the day. During tho 

 next day it again disappears entirely. A portion unites with lime to 

 form calcium malate, which is stored up as a reserve-material. 

 Another portion is re-transformed, by the action of light, into carbo- 

 hydrates. 



That the carbohydrates are really formed at the expense of tho 

 malic acid, is shown by the fact that the decrease in quantity of the 

 latter is accompanied by a corresponding increase in the amount both 

 of sugar and of starch, and that this is the case even in an atmosphere 



* Bot. Ztg., xliv. (1886) pp. 273-83, 289-98, 305-6, 329-38. 345-55, 361-6 

 (3 figs.)- 



t See this Journal, ante, p. 283. 



% Abhandl. Naturfgesell. Halle, xvi. (1886). See Naturforschcr, six. (1886) 

 p. 177. 



