ake BOTANICAL GAZETTE [ocroper 
where the forest has been undisturbed. Blueberry, barberry, black- 
berry, hackberry, huckleberry, raspberry, strawberry, currants, 
cornel, cedar, juniper, sumac, sassafras, black cherry, pin cherry, 
choke cherry, and black haw occur commonly in the dry sand of oak 
woods, less commonly with the hickories, and least of all in the 
maple-beech woods. Oaks, however, have thin crowns, are not 
shade-enduring, and do not grow crowded together, forming open 
woods that give a chance for the growth of a variety of herbs, shrubs, 
and vines on the forest floor. On the other hand, the beech, maple, 
and ash are distinctly shade-enduring, have thick crowns, and grow 
crowded together, forming a dense woods in which there is little 
chance for undergrowth to come in. In such woods there are s0 
few shrubs and vines that the average woodsman is not aware of their 
presence at all, and finds himself at a loss to account for the thick 
berry patch that springs up wherever he cuts a few trees. It is 
plain that seeds are scattered in all woods, but only the open woods 
afford a chance for their growth. 
6. THE HISTORIC FACTOR.—The presence of Sarracenia near cold 
seepage springs, and of the white pine on the bluffs, suggests that 
those plants may be relics of a past northern flora, which followed 
closely the retreat of the ice. While the tamarack society probably 
does not date back to this period in its origin, its flora may have been 
derived in large part from species so left, in the same way that ie 
juniper-heath spreads out over exposures as fast as made on the 
bluffs. The fact that 70 per cent. of the species associated are north- 
ern adds confirmation, and accounts for the dissimilarity between J 
and neighboring societies. With further investigation it does . 
seem improbable that many if not all of the peat bog forms may 
found lurking in the neighborhood of springs of this character. : 
Societies of the favorable places of the floodplain and bluffs apis 
a pronounced southern flora. A few northern forms occur = all 
floodplain societies; but the oak-hickory society, in which ao 
occurs a high per cent. of southern forms, has so few northern ese 
that their occurrence may be regarded as accidental. pee 
(fig. 5) shows how in land situations southern tendencies fade 0 
as conditions become more and more unfavorable. 
