84 GROWTH AND WORK OF PLANTS 



or American white hellebore (Veratrum viride) in swamps and damp woods. 

 The leaves are in three rows, and the arrangement is expressed by the frac- 

 tion J. The five ranked arrangement is shown in the apple, poplar, etc. 

 Here the spiral makes two revolutions for five leaves, and the fraction is f . 

 This study can be carried on in the winter by the leaf scars, though a study 

 during the growing season is preferable if it is possible. It will be found 

 profitable, if possible, to prosecute this study in the fields or parks, when 

 the students can make their observations and notes on opposite, whorled 

 and alternate leaves with a determination of the rank of the arrangement. 



5. RELATION OF LEAVES TO LIGHT. 



134. The light relation of leaves. One of the most impor- 

 tant relations of the leaf is its relation to light, because of the work 

 in the manufacture of sugar and starch (photosynthesis, Chapter 

 XIII). It will be seen that the various arrangements of leaves 

 shown in the preceding paragraph are of great importance in 

 giving them a suitable light exposure. This position on the 

 stem obviates the too great shading of adjacent leaves. A more 

 important relation, however, is the position which leaves take 

 in response to the stimulus of light. The position which leaves 

 occupy on the stem is governed largely by laws of growth in the 

 formative tissue in the bud. The position which the leaf blade 

 takes as it expands is in response to light stimulus. This posi- 

 tion, in general, is such as to bring the broad upper surface 

 of the leaf perpendicular to the stronger light rays so that the 

 work of the leaf which is carried on jointly with light maybe 

 most effective. 



135. Position of leaves with reference to light. Since the 

 stronger light rays, when we consider any considerable period of 

 time, come from the zenith, most leaves have in general a hori- 

 zontal or nearly horizontal position. But there are many condi- 

 tions which bring about a different result. On the edge of a 

 dense forest or clump of large plants, or where for other reasons 

 there is strong shade on one side, lower plants, unless they are 

 shade plants, have their leaves turned more or less so that they 

 face to one side to receive the incidence of the strongest light rays. 



136. Position of the leaves of "compass "plants. There 



