THE FACTORS OF DISTRIBUTION 39 



Xerophytes, on the other hand, may have long or 

 copious roots; corms, bulbs, tubers, or other forms of 

 fleshy underground stems, so frequent in the plants of 

 Cape Colony; a dwarfed habit, sometimes leading to 

 the formation of dense, cushion-like masses, as in the 

 Balsam-bogs of the Falkland Islands; or unbranched, 

 barrel -shaped stems, as in Baobabs and some Cacti; 

 branched and fleshy ones, as in Collstia, Opuntia, and 

 Phyllocactus ; or much -branched, whip-like, and nearly 

 leafless forms like our Common Broom. They commonly 

 develop spines and prickles, as is so strikingly charac- 

 teristic of the Acacias and Euphorbias of Africa and of 

 the Cactacea which " represent " the latter in America. 

 They commonly secrete resins, gums, and essential oils: 

 resins being markedly characteristic of the Conifera, 

 gums of the Acacias, and essential oils of the Labiates, 

 which abound in the Mediterranean region, and of the 

 Myrtacece, which include the Eucalypti of Australia. 

 Their leaves are generally persistent, thick, with a thick 

 cuticle, and few stomata, which are sunk below the 

 general surface of the leaf. They may be large and 

 succulent, as in the African Aloes and American Agaves ; 

 or smaller, as in the fleshy rosettes of the House-leek 

 (Sempervivum) and other CrassulacecB and many shore- 

 plants, with a thick water-storing hypoderm; dry and 

 rigid (sclerophyllous), as in Olives, Evergreen Oaks, 

 Proteacece, and the needle-leaves of the Coniferce ; with 

 inrolled margins, as in Heaths and the grasses of sand- 

 dunes; or delicate in texture but folding automatically 

 when exposed to strong sunlight, as in the compound 

 leaves of Mimosa and Oxalis. The flowers of xerophytes 

 are sometimes protected by numerous membranous 

 bracts, as in the Proteacece and " Everlasting-flowers " of 

 Cape Colony; and the whole surface is often densely 

 clothed with hairs or scurf-like scales. Xerophilous 

 vegetation characterises not only deserts and semi- 

 deserts or steppes, but such other situations as porous, 

 sandy, or calcareous soils, saline sea-shores, acid bogs, and 

 the cold soils of sub-arctic and alpine zones. 



According to their requirements as to temperature 

 and moisture, plants are divided into five classes: 



I. Hydromegathermic, in a temperature in which the 

 mean of the coldest month exceeds 1 6 C. ( = about 60 F. ) : 

 plants known to gardeners as " Stove-plants," now 



