174 



BIOLOGICAL LECTURES. 



of its sides. This is true, too, of the house begonia and many 

 others of our common plants. 



At high altitudes, where the differences of light and shade 

 are much more intense, these changes are correspondingly 

 more quickly brought about. With these external changes of 

 form there are microscopic differences which are even more 

 striking. In Plate No. lo are shown two cross-sections of 

 leaves taken from the Mountain Balsam (Abies frascri), a from 

 a sunny exposure and b from the more densely shaded part of 

 the tree. In Fig. a we have on the upper part of the leaf the 

 palisade cells above, and the more loosely arranged tissues, 



Fic;. a. 



Fig. b. 



Plate No. ro. 



for assimilation, below. About the middle of the leaf is a 

 laro-e resin duct. Fig. b is the cross-section of a similar 

 leaf from the same tree, carefully selected from a point 

 underneath the heavy branches which the direct sunlight never 

 reached. Above the resin duct, on the upper side of the leaf, 

 are the palisade cells, much as in a ; but the loosely arranged 

 assimilating tissue beneath the resin duct is seen to be absent ' 

 in /;. In other words, the light was not strong enough in the 

 lower shaded parts of this tree for assimilative purposes, and 

 in consequence these tissues have been dispensed with. 



(3) And here let me speak of the influence of altitude, which 

 affects not only the light but also the temperature and the air 



