EFFECT OF LIGHT ON UNDERGROWTH 157 



photographers use. Sensitive paper is exposed to the light until 

 it gets a certain shade, which is indicated by the side of the ex- 

 posed paper, the number of seconds being noted. In this way the 

 intensity of the light at the fringe of a wood may be compared 

 with the light of its shadiest part, and with that of intervening 

 areas. Then the herbaceous undergrowth of the same areas 

 should be compared. On May 23, observations were made by 

 a botany class of about ten girls on the undergrowth of a wood. 

 The part of the wood observed was carefully outlined in a 6-inch 

 ordnance map ; then the girls picked one of each species of the 

 herbaceous plants, and a complete list was made. This was done 

 in three different parts of the wood ; the number of species ob- 

 tained at the fringe of the wood was sixty, whilst that of the 

 inner part of the wood was only forty. The effect of the dense- 

 ness of the foliage of the trees composing the wood is well seen 

 in a beech wood, which should be carefully observed in early 

 spring, and the same area again in summer. The early flowering 

 plants, such as Bluebells, Wood Anemones, Lesser Celandines, will 

 often be very abundant ; but in summer, when the beech leaves are 

 fully out, there is hardly any herbaceous undergrowth, for no light 

 gets through. Then, plants which can do without light thrive, and 

 the Bird's Nest Orchis, or Toothwort, or Broomrape may be found. 



One effect of the difference in the intensity of light at the 

 fringe of a wood and in its thickest part is seen in the deeper 

 green of the leaves, and even in the petals of the flowers. There 

 is less chlorophyll in the leaves of a tree from the inside of a wood 

 than in those taken from the edge, and the difference in intensity 

 of colour is also seen in some of the herbaceous plants, though 

 less markedly in some than in others. 



Light is the principal, but not the only, factor affecting the 

 herbaceous undergrowth. The depth of humus, and conse- 

 quently the degree of moisture, is almost equally important. 

 The undergrowth of a Scots pine wood may be compared with 

 that of an oak from this point of view. To begin with, the leaves 

 of the Scots pine remain on the tree for over two years, and when 

 they do fall they decay but slowly, and form humus which soon 

 dries up. The undergrowth is therefore scanty. Where the 

 trees are close, giving deep shade, the Hair Moss (Polytrichum 



