RECENT ADVANCES IN SCIENCE 393 



Somerset, by W. Watson {Jour, of Ecology). Conspicuous 

 amongst the lower plants of the psamma dunes are the mosses 

 Tortula ruraliformis , Camptothecium lutescens, Brachythecium 

 albicans, and the lichens Cladonia furcata, Peltigera canina, 

 and P. rufescens. Greater fixity is, as with the phanerogamic 

 vegetation, marked by an augmented flora, with an increase 

 of Cladonia, and the presence of abundant Bryum spp. and 

 Barbula spp. In the fixed stages Hypnum cupressiforme , 

 Brachythecium rutabulum, and Hylocomium squarrosum become 

 prominent features. Riccia crystallina is a feature of the dune 

 " slacks," whilst the dune marsh proper exhibits Hypnum 

 aduncum and other species of the section Harpidia as the 

 cryptogamic dominants. 



PLANT PHYSIOLOGY. By Walter Stiles, M.A., University, Leeds, 

 and University College, London. (Plant Physiology Committee.) 



Irritability.— Very considerable interest has been displayed 

 recently in regard to irritability phenomena in plants, and 

 perhaps in no branch of plant physiology is such rapid develop- 

 ment taking place, especially on the Continent and in America. 

 A number of important contributions to this subject deal 

 with geotropism. Bannert (Beitr. z. allg. Bot. 1, 1-44, 191 6) 

 investigated the curving of the flower stalk and influorescence 

 axis in a number of different species. When the plants were 

 rotated on a klinostat with horizontal axis no curvature takes 

 place, from which it is concluded that the curvature is related 

 to a one-sided gravitational action. Pelargonium formed an 

 exception to the rule, for in this genus the influorescence axes 

 curved when rotated on the klinostat with horizontal axis. 

 Demole {Bull, de la soc. bot. de Geneve, (2) 8, 277-81, 19 16), 

 investigating the curvature of the developing fronds of bracken, 

 came to the conclusion that the change in direction of curvature 

 is due to a change from positive to negative geotropism of the 

 incurved region of the frond. This curvature was found to 

 take place only if the plants were growing in the light, and is 

 hindered by contact excitations. Demole investigated the 

 effect of these last, by stretching silk threads above the fronds. 

 When the fronds reached the threads the tactile excitations so 

 produced hindered the curvature change, but on removal of 

 the source of tactile excitation the change in curvature took 

 place in twenty-four hours. Provided the plant had been in 

 26 



