ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 487 



has fallen to a minimum. He further points out that Stahl's cobalt- 

 test * can only be reduced with great caution in estimating the amount of 

 transpiration from leaves. 



(3) Irritability. 



Conduction of Irritation in Plants.|— Dr. B. Nemec claims to have 

 detected, in the root of the hyacinth, a special tissue through which the 

 irritation-current is conducted, comparable to the nerve-fibrillas described 

 by Apathy in the Metazoa, and which he believes to exist in a large 

 number of vascular plants. The structure consists of rows of cells 

 running parallel to one another, and either merely in contact with one 

 another or actually in communication by threads of protoplasm. The 

 row of cells always lies in the direction in which the irritation is to 

 be conducted. In the youngest portion of the root-apex their direction 

 is usually nearly radial, in the central part longitudinal ; they are often 

 curved or twisted. In addition to protoplasm the cells contain starch- 

 grains, the position of which is very unstable. 



Geotropic Curvatures.J — As the result of a series of experiments on 

 nodes of grasses and roots of Vicia Faba, Mr. G. E. Stone has deter- 

 mined that the horizontal position is that of greatest geotropic excita- 

 bility, and that the relationship in this respect between nodes at oblique 

 angles and horizontal nodes is proportional to the cosines of their 

 angles. 



Periodic Movements of Plants.§— Miss D. F. M. Pertz and Mr. F. 

 Darwin state that if a sleeping plant is placed in a dark room after its 

 leaves have assumed the nocturnal position, its leaves will return on the 

 next morning to the diurnal position, notwithstanding the darkness. In 

 the same manner, if a plant is darkened after having responded to one- 

 sided illumination, it will return to the oblique position on awaking next 

 day in complete darkness. 



Paratonic Growth-curvatures in " Hinge-plants." || — By the term 

 "hinge" (Gdenk) Dr. F. G. Kohl describes what he calls a special 

 organ, that portion of a stem, always near a node, separated by an inter- 

 node from the nearest similar portion capable of movement, and placed 

 between two rigid portions of the stem. The "hinge-plants," or those 

 which possess such an organ, may be divided into two groups, those in 

 which the curvature movements are primarily and those in which they 

 are secondarily paratonic. In the first class the paratonic curvature 

 takes place during growth ; in the second it is the result of subsequent 

 external forces. 



In the species which was chiefly the subject of investigation, Trades- 

 cant i a viridis, it is usually the third or fourth node that is sensitive to 

 geotropic irritation. The internodes are not subject to geotropic irrita- 

 tion, but can conduct it, though only in the basipetal direction. The cells 

 in the neighbourhood of the hinge are characterised by the large amount 

 of sugar which they contain. 



* CF. this Journal, 1S95, p. 130. 



t Biol. Centralbl., xx. (1900) pp. 369-73 (1 fig.). 



j Bot. Gazette, xxix. (1900) p. 136. 



§ Proc. Cambridge Phil. Soc, x. (1900) p. 259. 



|| Bot. Ztg., lviii. (1900) 1" Abt., pp. 1-28 (2 pis. and diagrams). 



