98 



PHYLLOTAXY. 



[CH. VII 



goes thrice round the stem, and other arrangements are 

 similarly expressed. The phyllotaxy (f ) of the plantain is 

 shown in fig. 44. 



FIG. 44. 



PLANTAIN (Plantago), 

 viewed from above to show the f phyllotaxy : the leaf-spiral follows the 



course 1, 2, 3 13. 



From Le Maout and Decaisne. 



^07*771. 



The typical form of leaf is flat and thin ; that is to say, 

 although many plants have fleshy, cylindrical, or almost 

 spherical leaves, yet the majority have the form familiar 

 to everyone in the leaves of our ordinary trees. The 

 biological meaning of this form is plain enough, the leaf 

 being the assimilating organ which enables the plant to 

 build up organic material, it is necessary that a green 

 surface as large as possible shall be exposed to the light , 

 this will be realised when it is remembered how small 

 is the percentage of CO 2 existing in the atmosphere. 



