266 BRITISH PLANTS 
The undergrowth present depends upon : 
1. The amount of light coming from above. Some 
trees intercept much light and cast a deep shade— 
e.g., beech and sycamore—and under these the vegetation _ 
is scanty or absent ; others allow a considerable amount of 
light to penetrate through their loose crowns—e.g., oak, 
ash, birch—and this supports a rich and varied flora. 
both bushes and herbs. This is well seen in the accom- 
panying maps (Figs. 111, 112), taken from Woodhead’s 
paper on The Ecology of Woodland Plants, which show 
clearly that the distribution of the bracken is determined 
solely by the amount of light present, the soil varying in 
different parts of the wood from a stiff clay to a light sand. 
Fic. 111.—Mar or a4 Woop, sHowi1ne DistriBuTION OF TREES. (AFTER 
WoOoDHEAD.) 
Tone. ae = oak dominant. OO Fagus sylvatica. 
= © Ulmus montana. A Acer Pseudoplatanus. § A conifers. 
2. The nature of the soil, the presence of mild or sour 
humus, and the amount of water present. The soil-factor 
usually determines the kind of plant which is found, and 
the light-factor decides its abundance. 
The herbaceous or ground-vegetation in woods consists 
chiefly of shade-plants, their stems being tall or elongated, 
and their leaves often large. They are nearly all peren- 
nials, hibernating by rhizomes, tubers, or bulbs ; annuals 
are rare. In the humus of moist, shady woods a few 
colourless saprophytes grow (Neottia, Monotropa), but epi- 
