Manchester Memoirs, Vol. Ixii. (1918), No. 8. 3 



in Morocco. This applies to such genera as Ayperdeum, Cistus and 

 Ephedra. Some of the endemic forms have even a more distant 

 relationship. Ceropegia dichotoma and Dracana Draco, for example, 

 have their nearest relatives in the East, the former in India, the 

 latter in Socotra. This leads Engler, who bases his conclusions 

 largely on Hooker's account of the flora of the Canarian archipelago, 

 to consider this latter flora to represent in all probability the survival 

 of the flora of the Tertiary period, a flora of a more tropical and 

 Oriental character than the more xerophytic flora which to-day 

 characterises North Africa and the Mediterranean region. 



More puzzling is the occurrence of certain species which are 

 obviously related to similar species belonging to the American flora. 

 This applies to such genera as Per sea and Ocotea (Oreodaphne) . 



Factors which have influenced the Character of the Native Flora 



(1) Long isolation. Teneriffe has had no connection with the 

 African continent within any recent geological epoch. 



(2) Volcanic origin. The whole surface of the island is formed 

 of the products of igneous action ; basaltic and trachytic rocks, 

 recent or weathered lava-flows, pumice, cinders, ash, in places a 

 red loam of the nature of volcanic mud. 



(3) Climate : dry, warm and sunny, and free from extremes of 

 temperature. Rain, always insufficient in amount, falls between 

 mid-October and mid-April, the remainder of the year being, as a 

 rule, rainless. Steep slopes are everywhere the rule, so that rain 

 runs off rapidly. Cultivation is only rendered possible by an elabor- 

 ate system of irrigation. At the coast and up to moderate elevations 

 (Villa Orotava, Guimar, 300 m.) the lowest night temperatures in 

 winter do not fall below 8° C. (46 ° F.), while maxima in summer 

 rarely exceed 29" 5 C. (85 F.). At Orotava the mean winter 

 temperature is I7'5°C. (63 F.). 



Factors inducing Variations in the Distribution of the Plant Life of 



the Island 



(1) Great range in climatic conditions due to difference of 

 altitude, from sea-level to the summit of the Peak, 3760 m. 

 (12,192 ft.). 



(2) Connected to some extent with this, variation in annual 

 rainfall. The northern slope receives more than its share of the rain 

 brought by the north-eastern trades. Thus at Orotava the annual 

 rainfall is nearly sixteen inches, while at Guimar, on the southern 

 slope, it averages barely eleven inches. In winter the region of the 

 cloud-belt is frequently wrapped in drizzling rain, while the coast is 

 in full sunshine. Above the cloud-belt is another region of great 

 atmospheric dryness, that of the Peak and Canadas. 



(3) Variations in the substratum. While the greater part of 

 the island is covered by deposits due to eruptions of the Peak of 

 comparatively recent geological date, its two extremities, the pro- 

 montories of Anaga and Teno, are of much more ancient origin. 



