IV ON PLANT LIFE 143 



The "Compass" Plant of the American 

 prairies, a plant not unlike a small sunflower, 

 is another species with upright leaves, which 

 growing in the wide open prairies tend to point 

 north and south, thus exposing both surfaces 

 equally to the light and heat. Such a position 

 also affects the internal structure of the leaf, 

 the two sides becoming similar in structure, 

 while in other cases the upper and under 

 surfaces are very different. 



In the Yew the leaves are inserted close 

 to one another, and are linear ; while in the 

 Box they are further apart and broader. 

 In other cases the width of the leaves is 

 determined by what botanists call the " Phyl- 

 lotaxy." Some plants have the leaves oppo- 

 site, each pair being at right angles with the 

 pairs above and below. 



In others they are alternate, and arranged 

 round the stem in a spiral. In one very 

 common arrangement the sixth leaf stands 

 directly over the first, the intermediate ones 

 forming a spiral which has passed twice round 

 the stem. This, therefore, is known as the 

 f arrangement. Common cases are I, ^, f, f, 



