THE INFLUENCE ON FORM AND STRUCTURE 121 



that is when the water is too deep, no flowers are produced, even when 

 the floral primordia are normally developed beneath the surface 1 . 

 Similarly the conidia of Aspcrgillus nigcr and the sporangia of Mucor and 

 Pilobolus are not formed under water or within solid media 2 , whereas all 

 the reproductive organs of Saprolegnia are formed when the plant is 

 entirely submerged. Corresponding peculiarities are exhibited by terres- 

 trial and aquatic mosses 3 . 



An increase in the concentration of the external medium without 

 any change in its composition causes Vauchcria, Botrydinm, and Myxo- 

 mycetes to produce perennating bodies which are specially resistant to 

 drought 4 . Somewhat analogous to this is the fact that Phanerogams 

 which will not flower in moist habitats, do so when the supply of water 

 is reduced, possibly because of the lessened development of the vegetative 

 organs 5 . 



In many cases the change acts by inducing a modification of turgor, 

 which, however, only provides the means for stretching growth without 

 inducing it. The influence of transpiration in favouring the development 

 of the cuticle and of the conducting channels seems to be directly connected 

 with the movements of water it induces. Immersal in water necessarily 

 reduces the supply of oxygen, and this may be a factor of decisive 

 importance. 



Since, however, a tendril is able to distinguish between solids and 

 liquids, it is not impossible that the stem and leaves of aquatic plants can 

 perceive and respond directly to the gaseous or liquid condition of the 

 medium surrounding them. Moreover different parts may be dissimilarly 

 affected by changes of the medium, and in this way stimulatory responses 

 aroused. Since the result may be due* to the co-operation of a variety of 

 factors, it is often difficult to determine the precise origin of a particular 

 response. 



p. 367. According to Brand (Bot. Centralbl., 1894, Bd. LVH, p. 168), A'uphar hitcum forms no 

 floating leaves below 1.2 C. 



1 Cf. Schenck, 1. c., p. 112 ; Goebel, 1893, 1. c., p. 369. 



2 Klebs, Bedingungen d. Fortpflanzung, 1896, pp. 453, 472; Jahrb. f. wiss. Bot., 1898, Bd. 

 XXXII, p. 32 ; 1900, Bd. xxxv, p. 115 ; J. Ray, Rev. gen. d. Bot., 1897, T. ix, p. 257; Grantz, 

 Einfluss d. Lichts auf einige Pilze, 1898, p. 61 ; Werner, Die Bedingungen der Conidienbildung 

 bei einigen Pilzen, Dissert., 1898; Bachmann, Jahrb. f. wiss. Bot., 1899, Bd. xxxiv, p. 322; 

 Ternetz, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 298 ; Celakovsky, Bot. Centralbl., 1900, Bd. 

 LXXXIII, p. 292. 



3 Cf. Schenck, 1. c., p. 49; Lorch, Flora, 1894, p. 424; Goebel, Organography, 1900, Part i, 

 p. 261. 



* De Bary, Morphol. u. Biol. d. Pilze, 1884, p. 460 (Myxomycetes). On algae cf. Cienkowski, 

 Melang. biol. du Bull. d. 1'Acad. d. St.-Petersbourg, 1876, T. ix, p. 537 ; Rostafinski and Woronin, 

 Bot. Ztg., 1877, p. 660; Stahl, Bot. Ztg., 1879, p. 129 ; Klebs, Bedingungen d. Fortpflanzung, 1896, 

 pp. 223, 331, &c. ; Matruchot and Molliard, Compt. rend., 1900, T. cxxxi, p. 1248 (Stichococcus] ; 

 Livingstone, Bot. Gazette, 1900, Vol. xxx, p. 289. 



5 On the premature ripening of cereals see Frank, Krankheiten d. Pflanzen, 1895, Bd. I, p. 267. 



