86 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



are shown. The growth of certain organs is favoured by exposure to light, 

 and that of others retarded, while in some cases, though not in all, the 



stimulus of light is necessary for 

 the formation of particular organs. 

 A few instances of the more im- 

 portant of these light-reactions 

 must suffice. 



Etiolation. In addition to the 

 non - formation of chlorophyll, 

 plants grown in darkness usually 

 have longer internodes and 

 smaller leaves 1 . 



All plants do not, however, 

 react in the same way, and thus 

 the elongated leaves of Liliaceae 

 become longer and narrower in 

 darkness 2 , while the etiolated leaves 

 of Beta vulgaris attain a com- 

 paratively large size. The elonga- 

 tion of certain leaves in dark- 

 ness is, in certain cases, of impor- 

 tance in hastening the escape of 

 the leaf from the leaf-sheath, or 

 from the layers of soil which sur- 

 round it and cut off the light 

 from it. The elongation of the 

 internodes in darkness is of similar 

 importance in bringing the leaves 

 to light and air, especially when 

 bulbs, tubers, or seeds are buried 



PIG. 18. Potato plants grown from tubers. A in light, ^P 1 ? in the S ih TherC ' S > further > 



bered fr a r m n beiow^p e wardl. espondins: internodes &re num * an obvious economy in reducing 



as far as possible the size of the 



etiolated leaves, If these can only perform their natural functions when 

 exposed to light. 



The hypocotyls of seedlings whose cotyledons do not come above 



1 Sachs, Bot. Ztg., 1863, Beilage; Kraus, Jahrb. f. wiss. Bot., 1869-70, Bd. vu, p. 209; 

 Wiesner, Sitzungsb, d. Wien. Akad., 1893, Bd. en, I, p. 319; Ber. d. ]5ot. Ges., 1891, p. 46; 

 Rauwenhoff, Ann. d. sci. nat., 1878, 6 e s6r., T. v, p. 311 ; Kraus, Ueber einige Beziehungen des 

 Lichtes ztir Stoff- u. Formbildung, 1878 (Forsch. a. d. Geb. d. Agric.-physik, Bd. n); Godlewsky, 

 Biol. Centralbl., 1889, Bd. ix, p. 481; Ricome, Compt. rend., 1900, T. xxxi, p. 1251. The 

 leaves of certain conifers turn green in darkness, but others do not. Cf. Wiesner, 1. c., 1893, p. 344 ; 

 Jost, Jahrb. f. wiss. Bot., 1897, Bd. xxvn, p. 442. 



a For exceptions, see Walz, Bot. Jahresb., 3875, p. 787 ; Wiesner, 1. c., 1893, p. 319. 



