872 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Function of the Apex of Roots.* — G. Krabbe has further in- 

 vestigated the theories as to the cause of the motion of rootlets 

 advocated by Darwin and Wiesner. He states that the sensitive 

 portion of the apex of roots never exceeds 2 mm. in length, and varies 

 greatly below this limit. Since the portion of the root capable of 

 curvature is not included in this length, and amputated roots are still 

 able to grow, the author agrees with Darwin that the apex receives 

 from gravitation an impulse which it passes on to the zone of maximum 

 growth. He sujjports, however, Wiesner, as opposed to Darwin, in 

 his view that geotropism is a force entirely independent of circumnuta- 

 tion ; and that the latter is consequently not the cause of all the 

 movements to which roots are subject. 



Transpiration and Absorption of Water by Branches in 

 Winter, t — E. Hartig has obtained the following results from a series 

 of observations on transpiration and absorption of water on the part 

 of the bark or leaf-buds of a number of trees during winter : — The 

 transpiration both from angiosperms and conifers during winter varies 

 greatly with the species. With a normal quantity of water, i. e. 

 during the early days of the experiment, we get the following series, 

 commencing with the least rapidity of transpiration : — birch, oak, 

 copper beech, beech, black spruce, spruce, larch. On rainy days the 

 branches, especially of angiosperms, take up comparatively large 

 quantities of water. In autumn and the early part of winter the 

 branches remain constantly full of water, from which the tree must 

 obtain copious supplies. 



Hydrotropism.;!: — H. Molisch regards hydrotropism as a pheno- 

 menon of growth, and confirms Darwin's statement that a length of 

 from 1-2 mm. at the apex of the root is sensitive to psychometrical 

 differences, transferring the irritation to the growing part above it, 

 and causing curvature there. The hydrotropism of roots is only a 

 special case of the so-called " Darwinian curvature," depending on the 

 withdrawal of water from one side of the apex of the root ; the greater 

 dryness of the air on the side which becomes convex causes a stronger 

 transpiration of the adjoining half of the root-tip ; and this stronger 

 transpiration gives rise to the hydrotropic curvature. 



The rhizoids of Marchautiacese are positively hydrotropic ; while 

 both unicellular fungi, such as Mucor and Phycomyces, and many 

 multicellular fungi, such as Coprinus, are negatively hydrotropic. 



The tigellum is neither positively nor negatively hydrotropic, 

 even when the action of light and gravitation on one side only is ex- 

 cluded ; except in the case of the flax, the tigellum of which is 

 negatively hydrotropic. 



The author describes a new apparatus for observing hydrotropism. 

 A solid clay funnel provided with a perforated cover is allowed to dip 



* Ber. Deutsch. Bot. Gesellsch., i. (1883) pp. 226-36. Cf. this Journal, ii. 



(1882) pp. 529 and 531. 



t SB. Bot. Ver. MiJnchen, May 9th, 1883. See Bot. Centralbl., xv. (1883) 

 p. 92. 



X Aiizeig. K. Akad. Wiss. Wien, July 12th, 1883. See Bot. Centralb!., xv. 



(1883) p. 201. Cf. this Journal, ii. (1882) pp. 74, 530. 



