404 PHYSIOLOGY OF GROWTH. 



1 See Miiller-Thurgau, Landwirthschl. Jahrbucher,Bd. 11, p. 813. 



2 See Detmer, Pflanzenphysiologische Untersuchungen iiber Fermentbildung, 

 etc., Jena, 1884, p. 41. 



3 The results of A. Fischer's investigations also favour the views above brought 

 forward (see Jahrb. f. wissenschl. Bot., Bd. 22, pp. 127 and 154). Miiller- 

 Thurgau has recently taken a somewhat different view of the cause of the rest- 

 ing period in plants (Landivirthschl. Jahrb., Bd. 14, p. 878). See further 

 Askenasy, Botan. Zeitung, 1877. 



164. Growth of Plant Structures in Constant Darkness. 



Vigorous growth in constant darkness can naturally only take 

 place in structures which, even under these conditions, have at 

 their disposal considerable quantities of plastic material. Hence 

 seedlings are especially suitable for the following experiments, 

 because in their receptacles of reserve material larger or smaller 

 quantities of plastic substances are in fact always present. To 

 compare in a general way the behaviour of plants when growing 

 in constant darkness, with their behaviour when growing under 

 normal conditions of illumination, we place a few soaked seeds of 

 Pisum, Phaseolus, and Cucurbita in large flower-pots filled with 

 moist garden earth. Some of the flower-pots are placed at the 

 window, so as to be subject to alternation of day and night ; others 

 stand immediately by them under a large cardboard box covered 

 with black paper. It is well to conduct the experiments in a room 

 in which the plants are only exposed to diffused light, since under 

 the influence of direct sunlight the air in the cardboard box would 

 rapidly assume a very high temperature. It soon appears that 

 the two sets of plants, those developing in darkness on the one 

 hand, and those growing under normal conditions of illumination 

 on the other, present a very different appearance. Leaving 

 entirely out of consideration the absence of green colour in the 

 plants growing in the dark, we shall find, e.g., that in Cucurbita, 

 the hypocotyl has, in darkness, attained a very considerable 

 length, whereas in the illuminated plants it is still comparatively 

 short. The cotyledons of the shaded plants are, on the contrary, 

 neither so broad nor so long as those of the plants growing in the 

 daylight. By accurate measurements (several plants must al- 

 ways be examined in order to obtain a trustworthy average) we 

 can prove this in detail. In Fig. 132 is represented the aerial 

 part of an etiolated seedling of Cucurbita, and in Fig. 133 the 

 corresponding portion of a normal seedling. By growing seed- 



