628 Professor L. C. Mtall, [Feb. 18, 



are intermingled with sea-pink and sea-plantain. At the Lizard, 

 wherever the serpentine comes to the surface, ling ceases, and the 

 Cornish heath (Erica vagans) takes its place. 



Here and there we find among the ling the large-flowered heaths 

 with nodding pink or purple bells (Scotch heath, cross-leavei 

 heath). The leaves of these plants are much larger and thinner 

 than those of ling ; they are called " rolled leaves," because the edges 

 curve downwards and inwards, partly concealing the under surface, 

 which bears the stomates. All our native heaths agree in possessing 

 wiry stems, long roots and narrow evergreen leaves, with a glossy 

 cuticle and small transpiring surfaces. The tissues are very dry, 

 and burn readily even when green or drenched with rain. It is 

 possible by good management to set acres of heather in a blaze, even 

 in midwinter, with a single lucifer match. The heaths wither very 

 slowly when gathered, and change little in withering. 



Some of these features are characteristic of desert plants. Many 

 desert plants have reduced transpiring surfaces and hidden stomates. 



Fig. 6. — Transverse section of roller! leaf of cross-leaved 

 Heath {Erica tetralix). 



They often have very long roots, as was particularly obseived in the 

 excavations for the Suez Canal.* The leaves are often small and 

 crowded, the stems woody, much branched and tufted. Bright sun- 

 light retards growth, and green tissues hardly ever present a large 

 absorbing surface when they are habitually exposed to bright light. 

 Accordingly the young shoots and branches do not push out freely, 

 but try to hide one behind another. The tissues of desert plants may 

 be remarkably dry ; they are often, however, remarkably succulent ; 

 the plant either learns to do without water for a long time together, 

 or to store it up. 



It is not without surjn'ise that we learn how similar are the effects 

 of tropical drought and of Arctic cold. The facts of distribution 

 would in themselves suffice to show that our moorland heaths are 

 well fitted to endure great cold. Ling extends far within the Arctic 

 circle, though it seldom covers large surfaces there, and it rises to 



* Examples are quoted by Warming. ' Lehrb. d. okol. Pflanzengeographie/ 

 p. 198. 



