94 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



the case of fern prothalli. Furthermore, the buds on a shoot of Salix 

 or Popnlns develop mainly on the illuminated side 1 , while, according 

 to Kny -, the growth of coleochaete is most active on the exposed side. 

 The chlorophyllous assimilating branches of Canlcrpa*, and many other 

 algae, develop mainly on the side exposed to light, whereas in Polyponis, 

 and a few allied plants 4 , the spore-bearing hymenium develops on the 

 shaded surface. 



SECTION 25. The Beaction of a Phototonic Plant to Changes 



of Illumination. 



Exposure to light does not form an essential general condition for 

 growth, and beyond a certain varying intensity of illumination every plant 

 is ultimately killed. This intensity is not reached in nature in the case of 

 those plants which normally grow in full sunlight, whereas many shade- 

 loving plants are killed by a shorter or longer exposure to the sun. The 

 same applies to bacteria, certain species of which cease to grow in diffuse 

 daylight. It is indeed possible that organisms may exist which are so 

 sensitive to light, that they can only grow in complete darkness. 



The above facts have been determined by researches upon the fatally injurious 

 action of intense light 5 , but no exact experiments have been made upon the 

 maximal and optimal intensities for growth. Wiesner G observed a cessation 

 of growth in various seedlings when exposed to light of only 1,300-5,000 

 candle-power, but this was probably the result of the heat radiated from the gas- 

 flame, for no stoppage occurs in damp air or under water in the much more 

 intense sunlight, and Oltmanns (1. c.) observed that seedlings continued to grow 

 when they were exposed to electrical light of 500,000 candle-power. 



Within certain limits, a decrease in the illumination produces an 

 accelerated rate of growth in a phototonic plant, and an increase a 

 diminished rate. This effect has been observed on lower and higher 

 plants, on green and non-green plants, on positively and negatively 



1 Wiesner, Sitzungsb. d. Wien. Akad. ? 1895, Bd. CIV, I, p. 685. 



2 Kny, Ber. d. Bot. Ges., 1884, p. 93. 



3 Noll, Arb. d. Bot. Inst. in Wiirzburg, 1888, Bd. Ill, p. 472 ; Klemm, Flora, 1893, p. 472. 

 Cf. also Stahl, Jahrb. f. vviss. Bot., 1892, Bd. xxill, p. 339; Berthold, Jahrb. f. wiss. Bot.. 1882, 

 Bd. XIII, p. 673. 



4 Schulzer v. Miiggenburg, Flora, 1878, p. 122 ; Sachs, Arb. d. Bot. Inst. in \Viirzburg, 1879, 

 Bd. II, p. 252 ; Holtermann, Mykol. Unters. a. d. Tropen, 1898, p. 115. 



5 As regards algae cf. Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xm, p. 569; Ewart, Annals of 

 Botany, 1898, Vol. XII, p. 379; West, ibid., p. 33. The literature on bacteria is given by Fliigge, 

 Mikroorganismeu, 1896, 3. Aufl., Bd. I, p. 441, and by Migula, Syst. d. Bact, 1897, Bd. I, p. 361. 

 On yeast cf. Lohmann, Einfluss intensiven Lichtes auf die Zelltheilung von Saccharomyccs, Rostock, 

 1896, p. 71. 



6 Wiesner, Ueberdie heliotrop. Erscheinungen im Pflanzenreich, 1878, I, p. 37 ; 1880, II, p. 13. 

 See the abstract by Oltmanns, Flora, 1897, p. 20. 



