162 THE BOOK OF NATURE STUDY 



Violets, may under these circumstances be found in lanes ; or in 

 a pasture separated from the adjoining wood by a cart road, 

 some feet in width, if the trees overshadow the road sufficiently 

 to cast a deep shade over it. Cowwheat, Betony, and St. 

 John's-wort may occur in the part of the pasture nearest the 

 wood, which through its greater area abounds in the usual pasture 

 plants : Knapweed, Medick, Yarrow, Agrimony, Wild Carrot, 

 Flax, etc. 



EFFECT OF ALTITUDE 



Certain trees are found to belong to different altitudes. The 

 greater the height above the sea level, the more stunted the tree, 

 which may attain only the dimensions of a shrub. The willow, 

 birch, rowan, and ash are occasionally found as high as 1700 ft. 

 on the Pennine range, but they are very stunted. At about an 

 altitude of 1500 ft. a rapid stunting in growth begins, and the 

 tree soon assumes the shrubby habit unless artificially sheltered. 

 The oak is not often found in our islands at a greater height than 

 1250 ft., the birch reaches 1750 ft., and the Scots pine and other 

 coniferous trees 2000 or more. In the sub- Alpine region of north 

 Perthshire the chief trees are the birch, the larch, and the pine ; 

 at a somewhat lower level oak woods predominate ; whilst farther 

 south, in Midlothian, the beech thrives, and, though not an indig- 

 enous species, is to some extent ousting the oak on account of the 

 deep shade cast by its foliage and its greater rapidity of growth. 



The birch and Scots pine are often associated together in 

 forests ; they are seen in perfection in Sweden, where the trees 

 are closely planted and therefore grow to a greater height than 

 with us. The tall, straight trunks of the birch with their silvery 

 bark form a delightful contrast to the warm red colouring of the 

 pine. The birch will grow literally anywhere. It may be seen 

 on mountain sides clinging to rocks ; it is equally at home in the 

 humus of old woods, or by the sides of the pavement in suburban 

 streets. It is indifferent to heat or cold in the sub-arctic and 

 temperate climates, and is found as far north as Lapland. Like 

 the oak and beech, it is a catkin-bearing tree. The staminate 

 flowers are ripe by April and hang down, whilst the pistillate 

 ones of the same branch are above them and catch the pollen 



