492 ZONES AND REGIONS [Pt. Ill, Sect. II 



altescandens and Lomaria attenuata. Epiphytes are very abundant : so far as ferns 

 are concerned we have Hymenophyhaceae and Potypodiaceae ; of phanerogamous 

 epiphytes there is only one, and it is a tree, Rhetinodendron Berteroi, Hemsl., 

 one of the Compositae and a tree-destroyer, like epiphytic species of Clusia and 

 Ficus. It is not however exclusively epiphytic, and many species of trees and 

 shrubs that otherwise grow as terrestrial plants become occasionally epiphytic in 

 very humid situations. 



The vegetation on the ground is chiefly formed of ferns and cellular cryptogams ; 

 phanerogams, being represented by only about a dozen species, play in it a quite 

 unimportant part. 



In spite of the heavy rainfall, dripping-points are never formed here. Other char- 

 acteristics of the tropical rain-forest are also absent, such as plank-buttresses, cauli- 

 flory, water-storing calyces. 



2. THE XEROPHILOUS WOODLAND FORMATION OF 

 THE WARM TEMPERATE BELTS. 



As in the tropics, the xerophilous woodland of the warm belts of the 

 temperate zones, so far as it possesses the semi-tropical climate defined on 

 p. 446, will be referred to the two types, savannah-forest and thorn-forest. 

 Here also the savannah-forest is allied to the grassland formation and 

 frequently passes over into it ; whilst the thorn-forest takes precedence 

 when irregular precipitations interrupted by dry seasons render the con- 

 ditions unfavourable to grassland, and with increasing climatic dryness it 

 passes over into thorn-shrubland, and finally into the open desert formation. 



i. THORN-WOODLAND. 



Thorn-woodland appears as an edaphic formation in rain-forest districts, 

 and in grassland districts, on very permeable, dry, sandy soil ; as a climatic 

 formation also it however covers extensive tracts, in particular in the 

 interior of Argentina to the west of the pampas, where, according to 

 Lorentz, it occupies the most diverse kinds of soil — pampas loam, dunes of 

 gravel and sand, granite and limestone. The extensive thorn-woodland 

 of Argentina — the ' espinal-formation ' of Hieronymus — merits the name 

 of forest only in certain places, in particular in the eastern part of the 

 country; towards the west it becomes bush-like and scrub-like, and in 

 the west and south it gradually passes over into desert-formation. The 

 trees are very varied, but with few exceptions, for instance Aspidosperma 

 Quebracho, are characterized by stunted growth, scraggy ramification, light 

 crowns, and rich formation of thorns. Similar characters are repeated in 

 the shrubs. These include many Leguminosae, in the first place species of 

 Prosopis (Fig. 256), Acacia, Mimosa, to a less extent Gourliea decorticans, 

 the ' chanar '-tree, after which Grisebach named the whole formation * 



Lorentz describes the formation as ' monte,' i. e. forest. 



