84 



ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 51 



tris) is probably the species most sensitive to soil reaction. Its 

 leaves frequently become yellow between the veins when the soil 

 is above pH 6.7. This yellowing of foliage, called chlorosis, is 

 most commonly associated with iron deficiency in Illinois. Chlo- 

 rosis is described in detail in Illinois Natural History Survey Cir- 

 cular 46. 



The oaks are w^ell known for their interesting fruit, the 

 acorn. The various species of oak are usually divided into two 

 major groups, the white oaks and the black oaks. These two 

 groups are easily distinguished by their leaves. The white oak 

 group has leaves with rounded, bristle-free lobes. The black oak 

 group has leaves with bristle-tipped lobes that are not rounded, 

 or leaves that are entire with bristles at the tip. 



Of the 18 species of oak native to Illinois, most are relatively 

 large trees which grow to heights of 50-90 feet. The blackjack 

 oak (Q. marilandica) , commonly called scrub oak, is a small tree. 

 It reaches a height of 20-40 feet and may grow shrublike in ap- 

 pearance. It has an open, narrow, round-topped crown with 

 drooping branches. 



Pin oak (Q. palustris) (Fig. 78), used extensively for 



Fig. 78.— The moder- 

 ately large pin oak 

 grows more rapidly 

 than most other oaks. 

 Its leaves (inset) have 

 five to seven deep, 

 sharp-pointed lobes. 



