86 HOUSE, GARDEN, AND FIELD 



by human intelligence. We cannot tell why among our 

 British Composite weeds particular kinds enjoy a special 

 dominance, each in a site of its own selection (dandelion 

 and daisy in short grass, hawkweeds on railway cuttings, 

 thistles in ill-drained pastures, groundsel on road-sides, 

 and so on). There are scores of other Composites which 

 to the obtuse perceptions of man would have seemed just 

 as likely to prevail in these very spots. 



The adaptations of plants to cold conditions are com- 

 monly such as these : (i) the stature is low (arctic willows 

 and birches, for example, are only a few inches high) ; 



(2) the shoots dare not expose themselves to the bitter 

 wind, and branching freely without elongating, form 

 dense, almost solid bushes, especially in wind-swept places ; 



(3) the leaves are small, very numerous, clustered, some- 

 times hairy, often leathery, with dense cuticle, sometimes 

 succulent, often evergreen ; (4) the stomates may be 

 protected by being depressed beneath the general surface, 

 by a waxy bloom, or by the rolling of the whole leaf into a 

 hollow sheath ; (5) the tissues are often particularly dry, 

 and growth is slow ; (6) the flowers are often large and 

 bright- coloured. 



Many of these adaptations are found in the purple 

 saxifrage, which is prostrate, with small, tufted, succulent 

 leaves, hidden stomates, relatively large and bright flowers. 

 Close examination brings to light some interesting details. 1 

 The leaf is of small size, rarely exceeding 5 mm. in length 

 (I in.) ; it is elongate, widening towards the tip, and 

 fringed by fine teeth. The leaves are grouped into 

 4-ranked rosettes, which open imperfectly, and are thus 

 partially screened from light and air. The upper part of 

 each leaf, which is more exposed than the rest, is covered 

 by a dense cuticle ; the stomates are restricted to the 

 lower part, which is often sheltered by other leaves. Near 

 the tip of the leaf is a large water-pore, by which any excess 



1 The leaves of the purple saxifrage have been minutely described by 

 Lazniewski in Flora, 1896. 



