LEAVES 



603 



round on exposed rocks, and the stem leaves are nearly as narrow 

 in the shade as in the sun. Both in sun plants and in shade plants 

 there is a gradation from broad basal leaves to narrow stem leaves, the 

 chief difference being that at any given node the leaf of the sun plant 

 is narrower and thicker. If the terminal shoot is removed, lateral 

 shoots develop, their first leaves being round, regardless of conditions. 



Furthermore, any sudden change, even 

 that from shade to light, results in the 

 arrest of the terminal shoot and in the 

 development of lateral shoots with round 

 basal leaves (fig. 873). Thus, Campanula 

 appears to resemble Slum in that there is 



882 fc883 884 



FIGS. 882-884. Shoots of a mulberry (Morus), showing leaf variation ; 882, a vig- 

 orous vertical shoot, showing relatively entire leaves spirally arranged on the stem; 883, 

 a horizontal shoot with leaves somewhat lobed and all in one plane facing the light, as 

 a result of stem and petiole twisting ; 884, a shoot similar to 883 but with smaller and more 

 lobate leaves. 



a more or less fixed succession of leaf forms, any shock to the plant 

 causing "rejuvenescence." 



Variations comparable to those of Campanula are exhibited by Satureja gldbra 

 (figs. 985-988) and by various species of Arabis and Lechea. In Nicotiana (fig. 

 786) and in many similar plants there is a very gradual change from the base to 

 the apex of the stem, the leaves becoming progressively smaller, shorter, and thicker, 

 as well as more erect; the changes in transpiration from the base to the apex prob- 



