176 BIOLOGICAL LECTURES. 



drop down to a position in which the approaching light will be 

 at right angles to their surfaces. Plate No. 1 1 is a photograph 

 of Rhododendron maximum taken at an altitude of over 4000 

 feet. It developed itself entirely in the sunshine. It shows 

 the upper, or young leaves, erected vertically upward until 

 they are nearly parallel with the sun's rays during the hotter 

 portion of the day. The leaves below, which are more or less 

 horizontal, are more than one year old. At the end of the 



PLATE No. 11. 



first year, the upper leaves, having become tough and coria- 

 ceous, will assume a position parallel to the older ones below. 



In Plate No. 12 is shown a photograph of Rhododendron 

 catawbiense at the same altitude, but standing in a thoroughly 

 shaded spot. Although a different species, the action of the 

 leaves is the same. It will be seen that the young and growing 

 leaves above put themselves quickly in a position parallel with 

 the leaves below during development. 



The differences between the microscopic structures of the 

 leaves exposed to direct sunlight and of those wholly shaded 



