INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 283 



The curve XY of Fig. 132 shows that the greatest retardation occurs with violet 

 and ultra-violet light, this effect decreasing with longer wave-lengths until it is 

 minimal in the yellow region, about the D-line. Beyond this region, with still 

 longer wave-lengths, the retarding effect again increases. These facts furnish 

 an explanation of the differences between the phototropic responses brought 

 about by different qualities of light. The greater is the growth-retarding 

 effect of any given quality of light, the stronger is its phototropic influence. 

 Other conditions remaining unchanged, the most pronounced phototropic 

 influence is exerted by light that impinges perpendicularly to the surface of 

 the sensitive plant organ. 



Phototropism is of great ecological significance. Positive phototropic 

 responses bring the plant and its parts into the most favorable conditions of 

 illumination, and the negative responses of tendrils and aerial roots take these 

 organs out of the sunshine into the vicinity of surfaces to which they can 

 become attached, such as the surfaces of fences, walls, tree-trunks, etc. 



It has recently been shown that many plants possess special structures that 

 are supposed to act as organs of light-perception. 1 For example, the epidermal 

 cells of the leaves of Campanula persicifolia are characterized by condensing 

 lenses in their outer walls, these thickenings being impregnated with silicic acid. 

 These lens-like structures are somewhat similar to the lenses of animal eyes. 



It has been seen that temporary absence of light (as during the night hours) 

 and one-sided illumination, which brings about phototropic responses, are both 

 markedly effective in determining the rate of growth and the formal develop- 

 ment of plants, and it is now to be added that prolonged absence of light exerts 

 an even more pronounced influence. Plants grown in darkness are very differ- 

 ent from those exposed to the ordinary succession of day and night. Such 

 plants are said to be etiolated; they differ greatly in form but are primarily char- 

 acterized by having yellow leaves and white stems. 2 



In plants that do not produce stems in darkness (such as wheat), the dark- 

 grown leaves are longer and narrower than are leaves grown in light. In such 

 plants the leaf surface is generally greater when they are etiolated than when 

 they are grown in light. In plants that form stems in darkness, the internodes 

 are much longer in darkness than in light and the leaves remain rudimentary 

 in darkness. In this class belong the pea (Pisum sativum), the Windsor bean 

 (Viciafaba), millet (Panicum miliaceum), the potato (Solatium tuberosum), etc. 

 The scarlet-runner bean (Phaseolus multiflorus) is also one of this class; it is 



1 Haberlandt, G., Die Lichtsinnesorgane derLaubblatter. Leipzig, 1905. 



2 In this connection, see: Sachs, Julius, Ueber den Einfluss des Tageslichts auf Neubildung und Entfal- 

 tung verschiedener Pflanzenorgane. Bot. Zeitg. 21 : (Beilage; separately paged, 1-30). 1863. Batalin, A., 

 On the influence of light upon the structural development of plants. [Russian.] Dissertation. St. Peters- 

 burg, 1872. [Latest German paper located is the following: Batalin, A., Ueber die Wirkung des Llchtes 

 auf die Entwicklung der Blatter. Bot. Zeitg. 29: 669-686. 1871. For an account of a large amount of 

 experimentation upon the morphogenic influence of light see: MacDougal, D. T., The influence of light and 

 darkness upon growth and development. Mem. New York Bot. Garden, v. 2. XIII + 319 p. New York, 

 1903. On the influence of different lengths of alternating periods of light and darkness, see: Garner, W. 

 W., and Allard, H. A. Effect of the relative length of day and night and other factors of the environment 

 on growth and reproduction in plants. Jour. Agric. Res. 18 : 553-606. 1920. 



