METHODS OF DETERMINING TOLERANCE. ol 
acetosella) have their leaves made up almost exclusively of cells of 
the spongy parenchyma, while the leaves of extremely light-needing 
plants have hardly any cells of the spongy parenchyma, but are 
made up also exclusively of palisade parenchyma. Stahl’s experi- 
ments have also demonstrated that the structure of leaves of the same 
species varies with different light intensities; in the shade the leaves 
are made up chiefly of spongy parenchyma; in full light, of palisade 
tissue. The spongy parenchyma is adapted to weak and the palisade 
tissue to strong light. The leaves of beech, for instance, in light 
are characterized by a dark-green color on their upper surface and 
Fic. 5.—Cross section of a spruce needle. a, Grown in the shade; b, grown in full light. 
strongly developed palisade tissue; in the shade they have a light- 
green surface, and are made up almost exclusively of spongy paren- 
chyma. Leaves in light often have a crumpled surface and are fre- 
quently more densely covered with hair than leaves in the shade; the 
veins of leaves in light are also developed more strongly; and, 
finally, the thickness of the leaf is closely connected with the struc- 
ture, a shaded leaf being so thin and limp that it readily rolls itself 
into a small tube, while the leaf in light has a dense structure. (See 
figs. 4 and 5.) In Table 5 is given a comparative thickness of leaf 
tissues. ; 
