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 
# 
Fic. 54. Entire plant of dog- 
tooth violet (Erythronium) 
Hardly any aérial stem ap- 
pears, the flower stalk being 
sheathed by the leafstalks 
and arising from a deeply Fic. 55. Rosette of leaves of the common 
buried bulb. One fourth evening primrose, as seen at the end of the 
natural size 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.! The student will find it well worth while to observe 
the arrangement of alternate leaves, which are so placed, both 
* Leaves just unfolding from the bud, some of those exposed to tropical 
suns, and certain others (sect. 51) are exceptions. 
