6o2 



ECOLOGY 



light and carbon dioxid for synthesis. In the xerophytic leaf there 

 is a relatively large amount of cell division parallel to the surface, 



Figs. 876-879. — Leaf variation in Geum virginianum ; 876, a basal leaf from a winter 

 rosette; 877, a higher leaf from a winter rosette; 878, a leaf from the lower part of a stem; 

 879, a leaf from the upper part of a stem. 



resulting in dorsiventral enlargement, while in the mesoph3ftic leaf, sur- 

 face enlargement results from the extensive lateral development of the 



individual cells sup- 

 plemented some- 

 times by an increased 

 number of cell divi- 

 sions perpendicular 

 to the surface. 



Variation in the 

 leaf contour of land 

 plants. — The hare- 

 bell (Campanula 

 rotundifolia) has a 

 basal rosette of peti- 

 oled, round, or ovate 

 leaves, while the up- 

 per stem leaves are 

 narrowly lanceolate 

 or even linear (figs. 

 871, 872). Early 

 experiments related these leaf variations to light, the large basal leave.s 

 being thought to result from shade or diffuse light, and the narrow 

 upper leaves from intense light. However, the basal leaves are 



Figs. 880, 881. — Leaf variation in the black oak (Quercus 

 velutina); 880, an upper leaf, exposed to strong sunlight ; 88r, 

 a shade leaf from a lower branch of the same tree ; leaves re- 

 sembling 881 especially characterize basal shoots (suckers). 



