ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



589 



nomenon of irritation altogether analogous to heliotroiiism, and that 

 hence, in order to bring about thermotropic curvatures, the only factor 

 to be regarded is the direction in which the rays of heat, if of sufficient 

 intensity, strike the part of the plant in question. 



Curvature of Roots.* — J. Wiesner has made a further investiga- 

 tion of the " darwinian " and the geotropic curvature of roots, with the 

 following results: — 



1. The so-called " darwinian" curvature of roots, caused by injury 

 to one side of the apex, has a double character, a secondary curvature 

 taking place above the maximum zone of growth, while the primary 

 curvature is below it. 



2. The primary ciu-vature is the result of growth, the secondary 

 curvature simply of turgidity, the cells above the injured spot in- 

 creasing in length. If the root is decapitated, the zone above the 

 wound, within which the darwinian curvature takes place, is elongated, 

 the cell-walls becoming more extensible. 



3. The darwinian curvature combines with other paratonic 

 nutations, as for exami^le with geotropic curvature. Geotropism 

 frequently neutralizes the darwinian curvature. 



4. The entire growth of decajntated roots grown in damp media 

 is less than that of those that remain uninjured ; while the lower zone 

 of such roots nearest the apex undergoes great extension in consequence 

 of the increase of extensibility of the cell-walls. In decapitated roots 

 grown under water this pathological increase in length is so great 

 that the total growth of such roots is greater than of unmutilated ones. 



6. The decapitation of roots causes a diminution of the turgidity 

 of the cells ; and since geotropic curvature decreases with this 

 diminution, it follows that decapitated are less geotropic than uninjured 

 roots. 



Torsion as a Cause of the Diurnal Position of Foliar Organs.t— 

 According to O. Schmidt, light, by promoting the growth in length of 

 the shaded side of organs, can produce curvatures, but not torsions. 

 So-called heliotropic torsions arc due to the action of gravitation. 

 The ordinary diurnal position of leaves is a result of the combined 

 action of light and of gravity, the latter causing the torsion without 

 which the position could not be attained. 



Assimilative Power of Leaves.t — J. Sachs has carried out a series 

 of experiments on a number of plants growing in the open ground, 

 for the purpose of ascertaining the phenomena connected with the 

 formation of starch in the chloroidiyll-grains, and its disapi)caranco 

 under normal conditions of vegetation. The results arrived at arc 

 very remarkable, in showing the extraordinary rapidity with which 

 starch is formed and again disappears when the conditions of vegeta- 

 tion are favourable. The plan pursued was to remove the chlorophyll 

 by alcohol, and then employ the iodine test to determine tlie presence 

 of starch. Leaves may bo perfectly decolorized by first boiling in 



• Anzeig. K. Akarl. Wiss. Wicn, 1884. See Hot. Ccntialbl., xviii. (1884) p. 95. 

 t Bor. Dcutbcli. Dot. Gcrtoll., i. (1883) pp. 501-11. 

 I Arbeit. Bot. Inst. Wiirzburg, iii. (1881) pp. 1-33. 



