SPECIAL AND GENERAL 151 



the spores to escape when the air is dry. During moist weather the dead 

 involucral leaves of Carlina and Helichrysum bend inwards and prevent 

 the dispersal of the seeds, whereas when dry they bend backwards and 

 remove the hindrance to dispersal J . In addition the pappus of Compositae 

 expands during dry weather when dispersal is possible, and closes when 

 the air is moist, so that any soaring fruits overtaken by rain are soon 

 washed to the ground. The well-known Rose of Jericho (Anastatica 

 hierochunticd) affords a striking instance of drought causing the branches 

 to curl up into a ball enclosing the fruits. When the rains begin they 

 re- expand, the fruits dehisce and the seeds take root in the soil 2 . Many dry 

 capsules are also capable of hygroscopic expansion and contraction. 



Hygroscopic torsions are performed by the setas of Funaria and 

 other mosses 3 , as well as by the conidiophores of Peronospora and a few 

 other fungi *. This is especially marked in the beaks of the carpels of 

 Er odium gruinum^ which is often used as a hygrometer ; and these move- 

 ments, like those of Stipa and Avena, help the fruit to bore into the soil 5 . 



Changes of shape produced by the loss of water are only the result 

 of the removal of the imbibed water of the cell-wall when the cell contains 

 no free water. The collapse and wrinkling of the cell-walls of a dead 

 tissue when a portion of the water filling the cells is removed results, 

 according to Kamerling, Steinbrinck, and Schrodt 6 from the cohesion 

 and high breaking-stress of the diminishing volume of water, while its 

 adhesion to the cell-wall causes the latter to be drawn inwards and 

 crumpled. The aid of the external atmospheric pressure does not appear 

 to be necessary, since, according to Steinbrinck, the same phenomenon 

 is shown in a vacuum. When the water in the cells ruptures, air rapidly 

 penetrates the cell, according to Steinbrinck 7 , so that the air-pressure 

 is rapidly equalized within the cell. 



When dry organs are placed in moist air, no water appears in the 

 cavities of the cells so long as the formation of dew is avoided. Hence 



1 Dutrochet, Memoires, &c., Bruxelles, 1837, p. 236; Detmer, Journal fiir Landw., 1879, 

 Bd. xxvil, p. in. 



Ascherson, Ber. d. bot. Ges., 1892, p. 94. 



Wichura, Jahrb. f. wiss. Bot., 1860, Bd. II, p. 198; Goebel, Flora, 1895, p. 483. 



Cf. Zopf, Pilze, 1890, p. 86. 



Hanstein, Bot. Ztg., 1869, p. 526 ; F. Darwin, Trans, of the Linnean Society, 1873, 2nd ser., 



, p. 149; Steinbrinck, Bot. Ztg., 1878, p. 580. 



Vol. 



6 Kamerling, Bot. Centralbl., 1897, Bd. LXXII, p. 53; ibid., 1898, Bel. LXXlii, p. 472 ; Flora, 

 1898, p. 152. See also the summary in Bot. Ztg., 1898, p. 330; Steinbrinck, Festschrift fiir 

 Schwendener, 1899, p. 165; Ber. d. bot. Ges., 1899, pp. 99, 325; ibid., 1900, pp. 48, 217, 275, 286. 

 Steinbrinck (I.e., 1900, p. 219) suggests the term ' Schrumpfeln ' for crumpling caused by the 

 cohesion-mechanism, but a special term is quite unnecessary. 



7 Schrodt, Ber. d. bot. Ges., 1897, p. 100; Steinbrinck, I.e., 1900, pp. 275, 286. Cf. also 

 Claussen, Flora, 1901, p. 422. 



