Lae 
1903] FLORA OF NORTH CAROLINA 369 
White ash, umbrella tree. 
Sweet gum, sycamore. 
Locust, black cherry, sassafras. 
Cucumber tree, chestnut. 
Tulip tree. 
The vegetation of such a district, therefore, is in superposed 
layers or stories. The different levels at which plants grow is a 
direct response to the environmental conditions of light and 
‘Moisture. These stories or layers may be termed vegetal strata. 
It is evident that, as one ascends from the ground to the crown. 
of the dominant forest trees, the moisture content of the air 
decreases as the light increases. On the ground in the forest, 
mosses, ferns, and a variety of shade-loving herbaceous plants 
abound on the rotting timber, or on the mold. 
A relationship exists between the amount of available light 
and the character of the vegetation in the forest. Almost all 
of the plants growing in the shade can adapt themselves to living 
in the open, exposed to the full force of the sunlight. In fact, 
when the timber is removed, the grass-grown or wood-grown 
clearings show many woodland species competing with the plants 
which always at first take possession of such deserted areas. 
Few, however, can adjust themselves to loss of light. When 
deprived of a large amount of light by-the growth of the forest 
crown in density, only those species remain which are truly 
shade-loving. This difference of behavior explains why so few 
herbaceous plants are found beneath the dense shade of the hem- 
locks and rhododendrons. Mitchella repens,” Viola rotundifolia, 
Galax aphylla, Leptorchis liliifolia, Peramium pubescens, and 
Listera Smallii seem to be the more common plants tolerant of 
the shade of the forest. On the higher mountains, the conifer- 
ous forest acts in the same way, for when the dominant trees are 
removed, or the close crown broken, many herbaceous plants 
Spring up and cover the ground. 
Another noteworthy fact, which is of philosophic interest in 
7©Compare MacDouca., D. T., The influence of light and darkness upon 
growth and development. Memoirs New York Bot. Garden 2. 1902. 
*7 The nomenclature used is that of Britton’s Manual. 
