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TEXTBOOK OF PLANT PHYSIOLOGY 



and supplant their predecessors. The self-ignition of hay which 

 sometimes succeeds self-heating is a purely chemical process of 

 auto-oxidation of the material dried out by a high temperature. 



To the thermophilous organisms belong also some of the blue- 

 green algae of hot springs which, according to certain data, have 

 been found thriving at a temperature of 60 to 70° C, and even of 

 93° C. 



90. Effects of Light on Growth. The Daily Periodicity of 

 Growth. Formative Action of Light.— Contrary to temperature, 



light is not a factor necessary 

 for growth. Practically all 

 plants can grow in darkness, 

 while some plants, such as the 

 bacteria and many fungi, can 

 complete their whole develop- 

 ment in absolute darkness 

 without essentially altering the 

 character of their growth. 

 Higher plants, at least their 

 green organs, usually show in 

 the absence of light a rather 

 marked divergence from their 

 normal form and are deprived 

 of green color. Such plants are 

 said to be etiolated. 



Etiolated plants differ from 

 normal ones mainly by having 

 excessively elongated stems and 

 underdeveloped leaf blades (Fig. 120). In case, however, the 

 stem does not grow during the first stages of development, a 

 stretching of the leaves takes place. This faculty of elongating 

 its organs in darkness is of great importance in the life of a plant. 

 Finding itself deep under the ground, it stretches its shoots till 

 they have reached light and then begins to unfold its leaves. 

 Anatomically, etiolated plants are distinguished by tissues which 

 are very little differentiated and by the prevalence of primary 

 parenchyma. 



There is no direct connection between etiolation and the absence 

 of chlorophyll or the shortage of nutrition. Stretching in darkness 

 is observed in many fungi, for instance Mucor, Pilobolus, Coprinus, 



Fig. 120. — Etiolated and normal seed 

 lings of the horse bean. 



