216 THE MOVEMENTS OF WATER 



mentary vascular bundles of mosses, sufficient water cannot be transferred 

 through the stem to prevent the drying up which active transpiration tends 

 to produce 1 . In submerged phanerogams, in which no rapid transference 

 of water is necessary, the tracheal elements are rudimentary or few in 

 number, whereas in twining plants they are developed to a very marked 

 extent, for here large quantities of water must be conveyed through thin 

 stems for considerable distances 2 . 



Within certain limits restricted by the specific powers of the organism, 

 the development of the conducting system is favoured when an increased 

 demand is made upon it, and although no completely satisfactory and 

 conclusive experiments have as yet been performed, still the results 

 obtained by Kohl, by Hartig, and by Jost 3 , tend to show that increased 

 transpiration induces a more marked development of the wood cylinder. 

 The fact that the woody elements are unequally developed in the terrestrial 

 and aquatic forms of amphibious plants corresponds with this conclusion 4 . 

 A discussion of anatomical details and specific peculiarities is hardly 

 necessary here 5 , and hence no description need be given of the modes 

 of union between different vascular bundles, between successive annual 

 rings, between the primary and secondary wood of the root, between graft 

 and stock, between parasite and host plant 6 . Nor can the transition 

 tissue be described by which the transference of water between the vascular 

 bundles and the surrounding parenchyma is rendered possible 7 . 



The diosmotic properties of the cell-walls contiguous to the vascular 

 bundles are always of great importance, and frequently the pits on the 

 tracheal walls will form points at which the transference of water is 

 especially easy. Nevertheless it must not be forgotten that the other parts 

 of the wall are permeable also, and when the pits are few in number less 

 of the water may pass through them than through the remaining parts of 

 the wall (cf. Sects. 20 and 21). It is, for example, by no means certain 



1 Cf. Haberlandt, Jahrb. f. wiss. Bot., 1886, Bd. xvil, p. 374; Vaizey, Ann. of Bot, 1887, Vol. I, 

 p. 147. 



3 Westermaier u. Ambronn, Flora, 1881, p. 417 ; H. Schenck, Beitrage z. Biol. d. Lianen, 1893, 

 Bd. II, p. 6. 



:i Kohl, Transpiration d. Pflanzen, 1886, p. 116. Cf. also Wider, Bot. Zeitung, 1889, p. 549 ; 

 R. Hartig, Ber. d. Bot. Ges., 1888, p. 224; Bot. Zeitung, 1892, p. 176; Jost, Bot. Zeitung, 1891, 

 p. 546; 1893, p. 90. 



* Haberlandt, Physiol. Anat., 1896, 2. Aufl., p. 277; Constantin, Ann. d. sci. nat, 1884, 

 vi. ser., T. xix, p. 287 ; Schenck, Ber. d. Bot. Ges., 1885, p. 481. 



5 Details by de Bary, Comp. Anat., 1877; Haberlandt, Physiol. Anat., 1896; Strasburger, Bau 

 u. Verricht. d. Leitungsbahnen, 1891. 



6 Details by Gnentzsch, Flora, 1888, p. 309 ; Strasburger, 1. c., 1891, and Saftsteigen, 1893, p. 22 ; 

 Schwendener, Sitzungsb. d. Berl. Akad., 1892, Bd. xi.iv, p. 928 ; Jahn, Bot. Centralbl., 1894, Bd. Lix, 

 p. 360 ; Strasburger, 1891, 1. c., p. 503. Cf. Pierce, Ann. of Bot., 1893, Vol. vn, p. 291. 



7 In addition to the general works quoted above, cf. upon roots Siedler, Cohn's Beitrage, 1887, 

 Bd. v, p. 405. 



