PHOTOMORPHIC ACTIONS 87 



ground, undergo little or no increased elongation in darkness (Pisnm, 

 Aesctilus, Tropacolum], and the same applies to hop stems, to the spring 

 shoots of Dioscorea Batatas, and to the lower internodes on the stems of 

 Bryonia dioica (Sachs, Wiesner, I.e.). The absence of light produces, 

 however, pronounced changes in flattened chlorophyllous stems, and in 

 Phyllocactns and Opuntia this may go so far as to lead to the production 

 in darkness of stems with a more or less radial structure 1 . The same 

 thing happens in the assimilating aerial roots of certain orchids, which are 

 flattened when grown in light-, while the green aerial roots of Taenio- 

 phylhim Zollingcri only grow when exposed to light :! . On the other hand, 

 subterranean and aquatic roots acquire similar shapes in darkness and in 

 light 4 . 



Very weak continuous or intermittent illumination distinctly influences 

 the shape of an etiolated plant, and direct experiments, as well as compara- 



. 



FIG iq Transverse sections of leaves of Fagus sylvatica. A exposed to sunlight, B from a shaded locality. 



(After Stahl.) 



tive observations upon the same plant grown in sunny and in shady 

 localities 5 , suffice to show that the internodes become shorter and the 

 plant more condensed as the intensity of the light increases, while the 

 leaves attain their maximal size at a certain medium intensity of illumina- 

 tion G . This latter is owing to the fact that moderate light stimulates the 

 growth of the leaves, whereas intense light retards it. 



1 Vbchting, Jahrb. f. wiss. Bot., 1894, Bd. xxvi, p. 465; Goebel, Flora, 1895, p. 96; 

 Organography, 1900, I, p. 247. The etiolated shoot usually grows longer in darkness, although 

 the individual internodes are in part smaller than those formed in light. 



" Janczewski, Ann. d. sci. nat., 1885, 7 e ser., T. II, p. 55 5 Goebel, Organography, p. 246 seq. 



3 Wiesner, Sitzungsb. d. \Vien. Akad., 1897, Bd. CVI, I, p. 97. 



4 Nobbe, Versuchsst., 1867, Bd. IX, p. 80; Famintzin, Bot. Ztg., 1873, p. 366; Strehl, Unters. 

 ii. d. Langenwachsthum, 1874, p. 24; Walz, Bot. Jahresb., 1875, p. 787; Godlewski, Bot. Ztg., 

 1879, p. 91 ; Teodoresco, Ann. d. sci. nat., 1899, 8 e ser., T. X, p. 211. 



5 Other factors (transpiration, &c.) also enter into play. 



6 Sachs, Experimentalphysiol., 1863, p. 33; Bot. Ztg., 1871, p. 681 ; Stahl, Ueber d. Einfluss 



