220 



MISCELLANEOUS PUBLICATION 1271. U.S. DEPARTMENT OF AGRICULTURE 



seeds (for example, Begonia evansiana Andr.) 

 and short-day seeds {Veronica tournefortii C. C. 

 GmeL). 



The Etiolation Syndrome 



Dark-grown plants differ in several important 

 and easily recognizable ways from plants grown 

 in the light (fig. I). 2 Internodes of dark-grown 

 seedlings elongate more, while leaves and roots 

 are reduced in size, although monoeot leaves are 

 somewhat less inhibited than dicot leaves. Cole- 

 optiles of grass seedlings elongate rapidly in the 

 dark and are inhibited by light — adequately 

 serving their function of protection for the leaves 

 of the emerging grass seedling. Several dicot 

 seedlings have a hook near the tip of the epicotyl. 

 which straightens out upon exposure to light. 

 The symptoms of the etiolation syndrome extend 



-The figures used in tins paper are from (48), with 

 permission from the copyright owner. 



to the microscopic level, where chloroplasts de- 

 velop in response to light. Red light and the 

 photochrome system are effective in converting 

 an etiolated plant to a light-grown plant, but 

 other systems may also be involved. 



Plant Growth 



When the angiosperm seedling is exposed to 

 light, its protochlorophyll is converted photo- 

 chemically to chlorophyll (another feature of 

 the termination of the etiolation syndrome), and 

 the plant becomes autotrophic. Photosynthesis is 

 not usually thought of as a photomorphogenetic 

 process, but it supplies the energy for such pro- 

 cesses and thus strongly influences the forms and 

 sizes of range plants and others. 



Phototropism 



Plant organs typically become oriented in re- 

 lation to the direction of incoming light, stems 



Figure 1.— Etiolation in corn and beans; greenhouse-grown (left) vs. S days darkness (right). Note the following in 

 etiolated plants: Pale color, elongated stems, reduced leaves and roots, and hooks on bean seedlings. 



