44 SUMMARY OF CURRENT RESEARCHES RELATING TO 



cell-wall being at right angles to the incident rays. Similarly, the 

 rhizoids or holdfasts formed by germinating spores usually issue from 

 the daughter-cells away from the light. In darkness they arise in all 

 possible directions, sometimes even from both cells of a germinating 

 spore. The direction of growth of rhizoid and plantlet is determined 

 mainly by the direction from which the light comes : the rhizoids are 

 negatively, the plantlets positively, phototropic. The nature of the 

 surface largely controls the nature of the attachment formed, a rough 

 surface inducing the growth of a large and well-developed holdfast, 

 while a smooth surface causes proportionately less growth. Though the 

 direction toward which the rhizoids ordinarily grow is determined at 

 first by light, the character of the surface with which the rhizoid comes 

 into contact still more strongly influences the direction of its growth. 

 Thus the direction, rate, and kind of growth of these germinating 

 spores is strongly influenced by contact irritation. 



Galvanotropic Irritability of Roots.* — A. J. Ewart and Jessie S. 

 Bayliss, in view of the contradictory results obtained by Elfving and 

 Muller-Hettlingen, have reinvestigated this point. They have come to 

 the conclusion that the so-called galvanotropism of roots is really a chemo- 

 tropic phenomenon due to stimulation by the products of electrolysis, 

 of which the acid is more effective than the alkali, the latter also being 

 neutralised more or less by the respiratory carbon dioxide. That the 

 galvanotropic or galvanogenic curvatures are not traumatropic in origin 

 is shown by the fact that they may be produced without any cells being 

 killed. 



Chemical Changes. 



- The Chemistry of some Common Plants.f — Under this title P. 

 Keegan discusses the chemical constituents of Parmelia parietina and of 

 lichens generally. He points out a striking difference between lichens 

 and fungi ; that whereas the latter are frequently poisonous, the former 

 are always harmless, and also that their colouring matters are the products 

 of healthy de-assimilation, the colouring substances in fungi being mere 

 decomposition or degradation waste products of the albuminoids. The 

 writer looks on lichens as higher organisms than fungi, physiologically 

 speaking. 



General. 



Dispersal of Seeds by Wind.J — H. N. Ridley, of the Botanic 

 Gardens, Singapore, gives the results of his observations on this subject. 

 He recognises three types of seed-dispersal mechanism, namely, winged 

 fruits and seeds, plumed fruits and seeds, and " powder-seed," that is 

 dust-like bodies such as orchid-seed or fern-spores. He concludes that 

 the winged seed or fruit represents the slowest form of dispersal, and 

 cannot cross a large stretch of sea, while the plumed seed or fruit, 

 though well adapted for rapid carriage over open country, is liable to be 

 stopped by dense forest ; the powder-seed represents the most rapid and 



* Proc. Roy. Soc, Series B, lxxvii. (1905) pp. 63-6. 



t Naturalist, 1905, pp. 274-5. 



X Ann. of Bot., xix. (1905) pp. 351-63. 



