72 



INTRODUCTION TO BOTANY 



shrubs and trees, besides those already mentioned, are the 

 lilacs, the ashes, the horse-chestnut, and the buckeyes. Many 

 kinds of spirals occur, the simplest being that of indian corn, 

 in which the leaves stand in two rows along the stem, the 

 third directly above the first, the second 

 above the fourth, and so on. 



70. Importance of leaf arrangement. 

 The photosynthetic work of ordinary 

 leafy plants is run by sun power, or (to 

 put it more technically) is due to the 

 energy derived from the sun and acting 



FIG. 54. Entire plant of dog- 

 tooth violet (Erythronium) 



Hardly any aerial stem ap- 

 pears, the flower stalk being 

 sheathed by the leafstalks 

 and arising from a deeply 

 buried bulb. One fourth 

 natural size 



FIG. 55. Rosette of leaves of the common 



evening primrose, as seen at the end of the 



first year's growth of the plant 



on the green cells of the leaf. Clearly, therefore, it is usually 

 as important for sugar- and starch-making that the leaf should 

 catch all the sunlight it can get, as it is for rapid sailing that 

 a sailboat should expose its sails as fairly as possible to the 

 wind. 1 The student will find it well worth while to observe 

 the arrangement of alternate leaves, which are so placed, both 



1 Leaves just unfolding from the bud, some of those exposed to tropical 

 suns, and certain others (sect. 51) are exceptions. 



