132 



PLANT PHYSIOLOGY 



development in absolute darkness without essential alteration to 

 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 lacking in green color. 

 Such plants are said to be etiolated. 



Etiolated plants differ from normal ones mainly in having 

 excessively elongated stems and underdeveloped leaf blades 

 (Fig. 32). In case, however, the stem does not grow during the 



first stages of development 

 {e.g., in the case of grasses), a 

 stretching of 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 that 

 are very little differentiated and 

 by the prevalence of primary 

 parenchyma. Besides, they are 

 devoid of chlorophyll and are 

 usually of a pale yellow color. 



There is no direct connection 

 between etiolation and the 

 absence of chlorophyll or the shortage of mitrients. Stretching 

 in darkness is observable in many fungi, e.g., Mucor, Pilobolus, 

 Coprinus, etc., while very often no sporangium (Mucor aceae) or 

 pileus {Basidiomycetes) is formed, or else these are underdevel- 

 oped. Elongation in darkness is likewise pronounced in sprouts 

 of the potato, which are abundantly supplied with nutrient 

 substances from the tuber. In spite of numerous attempts to 

 explain the causes of the peculiar alterations in form of etiolated 

 plants, knowledge of the subject remains very scanty. Light 

 evidently is necessary to produce a normal cell structure and 

 tissue differentiation. Violet and ultraviolet wave lengths retard 

 cell elongation. Growth in an atmosphere saturated with water 

 vapor also leads to poor development of leaves and elongation of 



Fig. 32. — Etiolated (/) and normal (r) 

 seedlings of the broad bean. 



