LEAVES, THEIR FORM AND MOVEMENT ?? 



the spiral makes two revolutions for five leaves, and the fraction 

 is*. 



129. Practical studies on the arrangement of leaves. 



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: 



130. Position of leaves with reference to light. One of 



the most important relations of the leaf is its relation to light, 

 because of the work in the manufacture of sugar and starch 

 (photosynthesis, Chapter XII). 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 relation. 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 position, in general, is such as to bring the broad 

 upper surface of the leaf so that the stronger light rays will fall 

 perpendicularly upon it, since the work of the leaf which is 

 carried on jointly with light may be most effective. 



131 . Since the stronger light rays, when we consider any con- 

 siderable period of time, come from the zenith, most leaves have 

 in general a horizontal or nearly horizontal position. But there 

 are many conditions 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. 



