100 SYMMETRY. [CH. VII 



A striking feature in the typical leaf is that it is dor si- 

 ventral, it has a back and a front differing from each other ; 

 in the parts of plant$ hitherto considered this has not been 

 the case, the stem and the root are not dorsiventral but 

 are symmetrical round an axis. In leaves the dorsiventral 

 character is seen in a number of points, even in external 

 characters ; thus in many leaves the lower surface is paler 

 than the bright green upper surface, or it may be more 

 hairy or marked W projecting veins. The internal and 

 microscopic structure is even more plainly dorsiventral. 

 Connected with this character is a capacity of growth, a 

 sensitiveness to light by which leaves are enabled to 

 arrange themselves with one particular surface at right 

 angles to the light. To a plant growing freely in the open 

 air the light comes mainly from above, and thus it happens 

 that leaves are more or less horizontal, i.e. with the upper 

 surface at right angles to the vertical light. The surface 

 which thus receives most light, and which we usually call 

 the upper surface, is physiologically considered the- assimi- 

 lating surface. 



The power of adaptation is clearly seen in a plant so 

 placed that it receives light from one side, the leaves are 

 then twisted and tilted so as to make the most of the 

 light ; this may be well seen in a geranium growing in a 

 window, or a tropaeolum sprawling out of a flower-box. 

 Leaves vary extremely in shape ; what may be called the 

 typical form is well seen in the beech or the laurel, where 

 the stalk is continued as the midrib into the lamina, 

 which spreads out symmetrically on either side. From the 

 midrib a number of "veins" run right and left towards 



