122 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



A diminution of turgor retards growth, but some other factor must be 

 responsible for the fact that the fully turgid roots of terrestrial plants grow more 

 slowly in damp air or in water than in soil, whereas contact with soil retards 

 the growth of the roots of such aquatic plants as Lemna, Hydrocharis^ and Azolla x . 

 Wakker has shown that these results are not due to an insufficient supply of 

 oxygen, although this is in some cases responsible for the slower growth of the 

 stems of terrestrial plants when submerged in water 2 . 



In the case of Hydrocharis Morsus-ranae, Ranunculus sceleratus^ and Marsilia 

 quadrifolia, the petioles of the floating leaves cease to elongate as soon as the 

 lamina reaches the surface, while if the water-level is raised a corresponding 

 growth in length is induced in the petiole 8 . This purposeful reaction takes 

 place when the air above is fully saturated with moisture, and hence is not due 

 to any increase of transpiration, and consequent fall of turgor, nor has Karsten 

 succeeded in showing that it is due to an alteration in the supply of oxygen. 

 Covering the leaf with a thin film of water suffices to cause the elongation of the 

 petiole ; hence the latter can hardly be the result of the increased pressure of the 

 water, or of the increased tension on the petiole due to the upthrust of the water 

 on the leaf. [The upthrust of the water on the leaf is, in the absence of any 

 compression, independent of the depth of immersal. In the case of a water-lily 

 leaf of say 100 sq. cm. area on one surface, and 2 mm. thick, the increased tension 

 on the petiole when the leaf was submerged would be equal to 1 2 grammes if the 

 density of the leaf as a whole was 04, assuming that the tension is nil when 

 the leaf floats on the surface. On a petiole of 0-4 sq. cm. sectional area this 

 would give a tension of 30 grammes per sq. cm., which, though relatively small, might 

 easily evoke a stimulatory response in a sensitive organ, for it is frequently the 

 case that an applied tension accelerates growth. In any case the question is one 

 that can easily be answered by direct experiment.] 



The aeriferous system and air-bladders of aquatic plants are usually better 

 developed when the plant grows in water than when it develops in air 4 , and this 

 also applies to the local or general formation of the different forms of aerenchyma 5 . 

 On the other hand, ordinary aerial leaves undergo less anatomical differentiation 

 when development under water renders transpiration impossible 6 . 



1 Sachs, Arb. d. Bot. Inst. in Wurzburg, 1874, Bd. I, pp. 409, 589 ; Wakker, Jahrb. f. wiss. Bot., 

 1898, Bd. xxxil, p. 77. 



2 Frank, Cohn's Beitr. z. Biol., 1872, Bd. I, p. 76; Vochting, Organbildung, 1878, p. 131; 

 Wakker, 1. c. According to Maze* (Anfcal. d. 1'Institut Pasteur, 1900, T. xiv, p. 250), all seeds are 

 not able to germinate under water. 



3 Frank, Cohn's Beitr. z. Biol., 1872, Bd. I, p. 31 ; Karsten, Bot. Ztg., 1888, p. 566; Goebel, 

 Pflanzenbiol. Schilderungen, 1893, p. 311. 



4 Schenck, Jahrb. f. wiss. Bot, 1889, Bd. XX, p. 526 ; Goebel, Organography, 1900, Part I, p. 260. 



5 Schenck, 1. c., p. 526 ; Goebel, Pflanzenbiol. Schilderungen, 1893, p. 256 ; Wieler, Jahrb. f. wiss. 

 Bot., 1898, Bd. xxxi, p. 519; v. Tubeuf, Forstl. naturw. Zeitschr., 1898, p. 519; Devaux, Ann. 

 d. sci. nat, 1900, 8 e ser., T. xn, p. 221. 



6 Eberdt, Ber. d. Bot. Ges., 1881, p. 371; Kohl, Transpiration d. Pflz., 1886, pp. 94, 114; 

 Lesage, Compt. rend., 1894, T. cxvm, p. 255 ; Bonnier, Ann. d. sci. nat., 1894, 7 sen, T. XX, 

 P- 350; Junger, Bibl. bot., 1895, Heft 32, p. i. Also de Bary, Comp. Anat., 1884, pp. 511, 530; 

 Kohl, 1. c., p.*9i ; Keller, Biol. Centralbl., 1898, Bd. xvm, p. 241 ; Wollenweber, Bot. Centralbl., 



