308 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



present at least part of the time, as in nature. Stem elongation appears to be differ- 

 ently retarded by different ranges of light wave-lengths; for wave-lengths about like 

 those of sodium light (yellow) the retardation is least, notwithstanding the fact that 

 the greatest intensity of sunlight generaUy occurs in this region of the spectrum. 

 Retardation is greater with wave-lengths corresponding to red and green light, and it 

 is greatest with wave-lengths corresponding to violet light. 



Prolonged absence of light induces etiolation in ordinary plants. Etiolated plants 

 have yellow leaves. If they produce stems, these are white or yellowish and have un- 

 usually long, slender internodes, with only rudimentary leaves. The potato sprout 

 grown in darkness is an example of this kind of etiolation. Such plants as Bellis and 

 Sempervivum, which form rosettes under ordinary light conditions, produce long 

 stems in darkness, with spirally arranged, rudimentary leaves. The youngest inter- 

 nodes of twining plants, when subjected to natural light fluctuation, are usually in a 

 condition that closely resembles etiolation; they are therefore not greatly different 

 when grown in maintained darkness. Under ordinary conditions these shoots subse- 

 quently become green, and the leaves develop and become green in the usual way; 

 they are thus "normally etiolated" only when young. 



The etiolation of ordinary plants is prevented by an adequate supply of light that 

 has wave-lengths corresponding to blue and violet light. But etiolation generally 

 occurs if the plants are grown in red-yellow-orange light — that is, in sunlight from which 

 the shorter wave-lengths have been removed or very much weakened. This relation 

 of etiolation to light wave-lengths is not directly connected with carbohydrate 

 photosynthesis. In some cases, however, the behavior of etiolated leaves seems to be 

 related to the supply of soluble carbohydrates. 



It appears that etiolation is brought about through the operation of several condi- 

 tions, among which are to be mentioned: low transpiration rates and resultant modi- 

 fications in the absorption and distribution of water and salts, low carbohydrate 

 supply (especially to leaves), and absence of the direct effects of light, as such. It 

 seems certain that many essential photochemical processes occur in illuminated plants, 

 besides the one by which carbohydrates are formed, which alone has been much studied. 



Plants subjected to the natural alternation of day and night develop very differ- 

 ently according to the intensity of the light they receive in the daytime, as well as 

 according to the relative lengths of the day and night periods. Leaves that receive 

 only diffuse light during the daytime are generally thinner, with less palisade tissue, 

 than leaves of the same plant form receiving direct sunlight during their periods of 

 illumination. Shade-grown plants exhibit other characteristic differences from sun- 

 grown plants, and shade-grown leaves or branches are in many cases markedly differ- 

 ent from sun-grown leaves or branches of the same individual. • Plant forms differ 

 with respect to their light requirements and with respect to the intensity of light 

 they are able to bear; they may be classified roughly into shade plants and non-shade 

 plants. 



Lower forms of plants, such as fungi and bacteria, are influenced by light. Pilo- 

 bolus grows healthily only when exposed to diffuse light during the daytime. The 

 colorless bacteria (e. g., the typhoid bacillus) are killed by a brief exposure to direct 

 sunlight. Sunlight (especially the ultra-violet rays) is a potent influence in the puri- 

 fication of the water of rivers polluted by sewage. 



Ability to respond to one-sided illumination, the response being more rapid enlarge- 

 ment on one side of the responding organ than on the other, is called pholotropism, or 

 heliolropism. With positive pholotropism the organ bends toward the more strongly 



