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TEXTBOOK OF BOTANY 



The petioles of leaves which develop in the shade of other leaves may 

 continue to elongate for a longer period and the blade finally reaches 

 the plane of the leaves above. Both cell division and elongation take 

 place. Many of the details of these processes are still unknown. 



Fig. 75. Mosaics formed by the leaves of two species of maple {Acer maciophijl- 

 lum and Acer circinatiim) , Olympic Mountains, Washington. Photo by W. S. 

 Cooper. 



Turgor movements and growth movements. Distinctions between these 

 two kinds of moxements are sometimes attempted. The difficulty in 

 making a distinction lies in the fact that all growth movements involve 

 turgor effects. The distinction becomes clear onlv when movements 

 occur in those tissues in which all growth has ceased. Movements that 

 are due entirely to changes in turgor are reversible. Those that are due 

 to differences in growth may become permanent. 



There are also curvatures and movements in which the cell structure 

 determines the direction in which the plant organ curves or moves. These 

 curvatures occur in such plant organs as leaves and petals of flowers in 

 which the cells in the upper side differ from those in the lower. Such 

 plant organs are said to be bilateral, in contrast to organs that are 

 radially symmetrical. The movements may be purely turgor movements, 

 or a combination of turgor and growth. Among the more familiar exam- 

 ples are the daily opening and closing of certain flowers in relation to 



