WILD LETTUCE 45 



of structure between the two sides of the leaf 

 was also found to exist. 



It is unnecessary to point out all the rea- 

 sons for the conclusion finally reached, that 

 compass plants are endowed with an organi- 

 zation which enables the leaves, as the mid- 

 day sun becomes unpleasantly bright, to turn 

 part way around and present less surface to 

 the action of its rays. Figuratively, one 

 might say the plant turns a cold shoulder to 

 the sun, when he becomes too ardent. 



Ordinary leaves permit the sun to shine up- 

 on the upper surface only, having that side 

 constructed to bear the light and heat with- 

 out injury, while the under side, having a 

 more delicate organization, is turned from the 

 sun. When the compass plant adjusts its 

 leaves in the only position possible by which 

 equal illumination is secured for the two sides 

 during the middle of the day, the under side 

 of the leaf is evidently in danger of injury un- 

 less reorganized. Such a change does in fact 

 come about. As the palm of the hand is cal- 

 loused by repeated rough usage, so the lower 

 surface of the leaf is inured to the sun's action 

 by exposure, the change consisting of a pro- 

 found alteration of the underl^ang tissues as 

 well as of the superficial portion. Physiolog- 

 ically there is no longer an upper and a low^er 

 surface to the meridional leaf, but simply a 



