1902] CURRENT LITERATURE 379 



tions on Pertiisaria co7?imu7tis. Krabbe who studied the same lichen states 

 that he found no trace of sexual organs or of a differentiation of ascogons 

 and paraphyses. Baur's sections show the characteristic thick, coiled 

 ascogons and trichogynes and the later developed ascogenous hyphae most 

 sharply differentiated from the surrounding vegetative tissue from which 

 arise the paraphyses. Of special interest in this lichen is the capacity for indef- 

 inite development shown by the ascogenous hyphae. Growing and branch- 

 ing richly at their tips and dying off at their basal ends as they advance, 

 they spread through the thallus and form apothecia at various points, even 

 2 ^^ distant from the original trichogyne and ascogon from which they arose. 

 Such extended independent growth of the ascogenous hyphae forms a good 

 parallel to what Watnio has already claimed for Cladonia. Baur also 

 describes the development of the pyrenolichen Pyrenula nitida, and finds 

 here also the characteristic differentiation of ascogon and trichogyne. The 

 recent work of Baur and others has made the morphology of the ascocarp for 

 the entire family of the lichens better known than it is in any other similar 

 series of Discomycetes. — R. A. Harper. 



Since the appearance of Czapek's answer to Wachtel's paper on the 

 method of bent tubes to demonstrate the sensitiveness of root-tips to the 

 gravity stimulus, we have had a desire for some entirely different method by 

 which this long-discussed question could be attacked anew. Czapek's posi- 

 tion seemed to be established, but the lack of confirmatory evidence from 

 other sources has been still noticeable. In a recent paper by Francis Dar- 

 win^ a new method of approach is described. It is a modification of that 

 used by him ^° in showing that the cotyledon of Setaria, Sorghum, etc., is the 

 perceptive region in geotropic curvature of the hypocotyl. Since the root tip 

 is slimy and the whole organ mechanically weak, it is impossible to fix the 

 tip in a horizontal tube and have it support the weight of the rest of the 

 seedling. The new method obviates this difficulty by affixing the cotyledons 

 to the end of a long lever free to move in both a vertical and horizontal 

 plane. The lever is of course counterbalaned, and thus the cotyledons (of 

 beans in this case) are able to move freely in any direction in a spherical 

 plane, whose radius of curvature is the length of the supporting lever-arm. 

 When the cotyledons are so supported the root-tip is placed in a horizontal 

 tube (of straw, dandelion scape, etc.), and complete turns are executed by the 

 curving root. The method is difficult of operation, and a large number of 

 experiments failed because the root-tip slipped from the tubes. But the author 

 believes he has demonstrated in this way that as long as the tip is horizontal 

 the response of the growing region produces a continuous curve resulting in 



^Darwin, F., On a method of investigating the gravitational sensitiveness of 

 root-tip. Jour. Linn. Soc. 35:266-274. figs. i~io, 1902. 



"Darwin, F., On geotropisra and the localization of the sensitive region. Ann. 

 Eot. 13:567-574-/^-^^. 1899. 



