EXERCISE 25 



HOW IS EXPOSURE TO LIGHT FAVORED BY LEAF 

 ARRANGEMENT ? 



Materials. The exercise is preferably worked out as a field trip, 

 but potted plants may be used in part if necessary. 



Directions for work. In studying the arrangement of leaves 

 with reference to light, remember that light comes from all 

 parts of the sky as well as directly from the sun's disk. The 

 average direction of light may therefore be taken as approxi- 

 mately vertical, but it must be remembered that much light 

 comes from other parts of the sky as well. 



1. Select a vertical stem which is not shaded or crowded by 

 any neighboring stems. This may be found on a small tree or 

 shrub, or a weed, such as ragweed, may be used. 



Beginning at the top, note the position of each leaf blade. 

 How many leaf blades face a part of the sky which is within 

 45 of the vertical ? How many of the leaves, when viewed 

 at right angles to the surface of the blade, appear to be shaded 

 by other leaves ? What proportion of the whole leaf surface of 

 the stem is so shaded ? 



2. Select a horizontal stem on either a creeping plant or a 

 nearly horizontal branch of a tree. Note the position of each 

 leaf and its exposure to light. In this case are the leaves dis- 

 tributed all around the stem, as was true of the vertical stem ? 

 If not, are they in this case attached to the stem in different 

 positions from the former example, or by what means are they 

 able to assume the new positions ? If the previous study (1) has 

 been made on a tree, it is interesting to use for this study 

 (2) another branch on the same tree, making comparisons be- 

 tween them. 



3. Examine a vine growing on a wall, as the Boston ivy. 

 Note that in this case the light from one half of the sky is cut 

 off by the wall. The average direction of the light reaching 

 the leaves is therefore oblique. 



Supposing the light to come to the leaves at an angle of 45 

 from the vertical, how large a part of the leaf surface appears 



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