ANATOMY AND PHYSIOLOGY 



tif ully visible with the returning branches of milky veins 

 on each side." 



Darwin now goes on to draw an incorrect inference 

 from his observations: 



"3. From these experiments, " he says, "the upper 

 surface of the leaf appeared to be the immediate organ 

 of respiration, because the colored fluid was carried to 

 the extremities of the leaf by vessels most conspicuous 

 on the upper surface, and there changed into a milky 

 fluid, which is the blood of the plant, and then returned 

 by concomitant veins on the under surface, which 

 were seen to ooze when divided with scissors, and 

 which, in Picris particularly, render the under surface 

 of the leaves greatly whiter than the upper one." 



But in point of fact, as studies of a later generation 

 were to show, it is the under surface of the leaf that is 

 most abundantly provided with stomata, or "breath- 

 ing-pores." From the stand -point of this later knowl- 

 edge, it is of interest to follow our author a little 

 farther, to illustrate yet more fully the possibility of 

 combining correct observations with a faulty inference. 



"4. As the upper surface of leaves constitutes the 

 organ of respiration, on which the sap is exposed in the 

 termination of arteries beneath a thin pellicle to the 

 action of the atmosphere, these surfaces in many 

 plants strongly repel moisture, as cabbage leaves, 

 whence the particles of rain lying over their surfaces 

 without touching them, as observed by Mr. Melville 



97 



