120 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



immediately germinating on such solutions, a power which is only gradually 

 lost when the cultures are continued on dilute media 1 . 



The rate of growth of terrestrial plants varies considerably according 

 to the amount of transpiration and to the supply of water, and in addition 

 to the direct action by influencing the percentage of water, Godlewski 2 

 observed a transitory disturbance of growth when the rate of transpiration 

 was suddenly increased. The optimal percentage of water for growth is 

 attained when the plant is fully turgid, for if the aeriferous intercellular 

 spaces of terrestrial and also of many aquatic plants are filled with water, 

 the aeration of the tissues is impeded, and growth, respiration, and 

 metabolism in general are retarded 3 . Most marine algae are unable to 

 develop when the osmotic concentration of the saline medium falls below a 

 certain limit, possibly partly because of the tendency of their gelatinous walls 

 to undergo excessive absorption of water and swelling when insufficiently 

 checked by the surrounding medium. 



SECTION 34. The Influence on Form and Structure. 



The supply of water not only influences the distribution of plants 4 , 

 but also affects the formation of cuticle and the development of the con- 

 ducting channels. The diminished growth produced by a deficiency of 

 water is of importance by keeping the exposed surface area of the plant 

 as small as possible, and hence reducing the loss by transpiration 5 . 

 Xerophytic characters induced in this manner may be more or less 

 completely lost when the plant receives an abundance of water. The 

 power of adaptation is, however, not always sufficiently great to lead to 

 pronounced changes of shape and structure, although in many amphibious 

 plants, such as Ranunculus fatitans, Sagittaria^ and certain algae, the 

 terrestrial and aquatic forms are so distinct as to appear like different 

 species. Similarly in many aquatic plants the parts above water acquire 

 different shapes to those beneath it. Thus the floating leaves of 

 Ranunculus aquatilis^Nuphar lutcum, Potamogeton natans, and the aerial 

 leaves of Sagittaria differ widely from the submerged leaves, which alone 

 are formed so long as the plant is under water 6 . Under such conditions, 



1 Errera, Bull. d. 1'Acad. royale d. Belgique, 1899, p. 95. 



2 Godlewski, Anzeig. d. Acad. d. Wiss. zu Krakau, 1890, p. 170. 



3 \Vollny, Forsch. a. d. Gebiete d. Agriculturphysik, 1897, Bd. xx, p. 56; Ad. Mayer, Jour. f. 

 Landw., 1898, p. 167. 



4 Cf. Schimper, Pflanzengeographie, 1898, p. 3. 



s Cf. Sorauer, Bot. Ztg., 1873, p. 145; 1878, p. i ; de Vries, Landw. Jahrb., 1877, Bd. vr, 

 p. 896; Frank, Krankheiten d. Pflanzen, 1895, 4- Anfl., Bd. I, p. 272 ; \Vollny, Forsch. a. d. Gebiete 

 d. Agriculturphysik, 1897, Bd. xx, p. 56, and the literature there given. 



6 Askenasy, Bot. Ztg., 1870, p. 193; Schenck, Biol. d. Wassergewachse, 1886; Goebel, 

 Pflanzenbiol. Schild., 1893, II, p. 283; Organography, 1900, Part I, p. 260; Wachter, Flora, 1897, 



