1898.] on a Yorkshire Moor, 629 



2000 metres (6600 feet) on the north side of the Alps. It extends 

 soutliward to the shores of the Mediterranean. Our large-flowered 

 heaths have not been traced quite so far north as ling, and they are 

 not found on the Alps, though they inhabit the Pyrenees. Many 

 representatives of the heath family, with like structure of leaves, are 

 found in the extreme north of the American continent. Those 

 features which assimilate our heaths to desert plants, and which 

 seem to be obvious adaptations to a situation of extreme drought, are 

 equally serviceable to plants which have to face boisterous winds and 

 low temperature. The shrubs of the far north are low, tufted, small- 

 leaved, evergreen and dry — just like the heaths of our moors. 

 Middendorff* shows how the Dahurian larch becomes stunted in 

 proportion to increasing cold. Before it disappears altogether, it is 

 cut down to a prostrate creeping shrub. One such dwarf larch, 

 though 150 years old, was only a foot or two across. Plants much 

 exposed to biting winds must make the most of any shelter that can 

 be had ; their branches push out timidly, and for a very short 

 distance ; the leaf surface is reduced to a minimum ; since the warm 

 season is short, evergreen leaves are profitable, for they enable the 

 j^lants to take advantage of early and late sunshine. 



The heaths and many other moorland plants bear the marks of 

 the Xerophytes, or drought plants. Xerophytes grow under a con- 

 siderable variety of conditions, some of which do not suggest drought 

 at first sight, but their tissues are always ill'Supplied with water. 

 It may be that w^ater is hardly to be had at all, as in the desert ; or 

 that water must not be imbibed in any quantity because of low 

 temperature, as in Arctic and Alpine climates ; or that the water is 

 mixed with useless and perhaps injurious salts, from which it can 

 only be separated with great difficulty, as in a salt marsh. Whatever 

 may be the reason for abstinence, xerophytes absorb water slowly^ 

 part with it slowly, and endure drought well. 



In the case of moorland plants there is an obvious reason why 

 many of them, though not quite all (Sphagnum is one exception) 

 should rather thirst and grow slowly than pass large quantities of 

 water through their tissues. The water contains hardly any potash 

 or lime, and very little that can aid the growth of a plant. But it is 

 probable that this is not the sole reason. Except where special 

 defences are provided, it is dangerous for a plant which may be 

 exposed to wind or low temperature to absorb much water. 



The Bilberry (or Blueberry, as w^e ought to call it) is one of the 

 few exceptions to the rule that moorland plants are evergreen j it 

 casts its leaves in early winter. But the younger stems are green, 

 and take upon themselves the function of leaves when these are 

 absent. Keruer has described one adaptation of the bilberry to 

 seasons when water is scarce. Many plants, especially those of hot 

 and wet climates, throw off the rain water from their tips, and so 



* ' Sibirische Reise,' vol. iv. p. 605. 

 Vol. XV. . (No. 92.) 2 t 



