i8 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



die off, become brittle, and break by reason of their weight or of storms ; 

 it is because of their suppression that the central parts of trees and shrubs 

 are leafless and have so few twigs. A spruce standing out in the open 

 is conical and bears branches from its summit to its base ; whereas one 

 standing in a dense forest has only a small green crown, and outside this 

 no branches, or only leafless dead ones, because its illumination is different. 

 Dicotylous trees, such as the oak or beech, standing in the open, have 

 a full ovoid or conical crown, but when growing in dense woods have 

 a small crown with upwardly directed branches.^ 



Light plays an important part in the struggles between trees that are 

 growing in company. Forest trees may be divided into 



(a) Light-demanding trees, which demand much light and endure but 

 little shade ; 



(&) Shade-enduring trees, which are content with less light and can 

 endure deeper shade. 



The reasons for these distinctions must be sought for in the specific 

 distinctions in the chlorophyll, rather than in any difference in the archi- 

 tecture (structure of the shoot, phyllotaxy, and form of the leaf) of the 

 species. Arranging our commonest forest-trees in accordance with their 

 demands for light when individuals of the same age are competing with 

 one another, we arrive at the following series, the order of which approxi- 

 mately denotes decreasing requirements as regards light : 



1. Larch, birch, aspen, alder. 



2. Scots pine, Weymouth pme, ash, oak, elm, sycomore. 



3. Pinus Montana, Norway spruce, lime, hornbeam, beech, silver fir. 

 It is worthy of note and biologically important that nearly all trees 



can endure deeper shade in early youth than they can later in life. It 

 may be added that the power to endure shade also depends upon the 

 fertihty of the soil. 



Distinction between Sun-plants and Shade-plants. 



Between heliophilous or photophilous plants, which prefer sunlight, and 

 heliophohous or sciophilous plants, which prefer the shade, there are 

 great differences in external form and internal structure. 



I. Intense light retards the growth of the shoot ; consequently, helio- 

 phytes are compact and have short internodes, but sciophytes have 

 elongated internodes ; species clothing the forest soil are mainly tall 

 and long-stemmed. The leaves of heliophytes are often small, narrow, 

 of linear or some similar form ; but those of sciophytes under the same 

 conditions are large and broad, longer in proportion to the width,^ and 

 thinner. The leaves of Maianthemum Bifolium in sunlight attain scarcely 

 one-third of the size that they reach in the shade.^ 



The leaves of many species, especially of cultivated plants, are larger 

 in northern lands than in lower latitudes ; this is possibly due to the 

 poverty of the light of high latitudes in rays of short wave-length. In 

 gardens on the west coast of Norway, for instance, the flowers of Tro- 

 paeolum ma jus lurk almost hidden beneath the mass of large leaves.* 



* See Vaupell, 1863. ^ Warming, 1901. * Kissling, 1895. 



* Bonnier et Flahault, 1878 ; Schiibeler, 1886-8, and others. 



