RHINANTHOIDEiE : SANTALACE.E 221 



requirements is least. Structurally these plants show 

 practically none of the reduction which is so marked a 

 feature of more advanced parasites. We will return to this 

 group later. 



We may here note the parasitism of Krameria canescens, 

 a desert shrub described by Cannon (19 10). It is a partial 

 root parasite on such small trees as Parldnsonia, and its 

 behaviour is much like that of Rhinanthus, though differences 

 in the details of the suckers exist. 



Santalacese. — At the same level of parasitism are the 

 semi-parasitic members of the Santalaceae. The family is 

 widespread in the tropics and sub-tropics, chiefly in dry 

 regions ; a number of genera occur in the Mediterranean 

 region, and one, Thesium, has several species in Central 

 Europe, and one in England. Some are small trees, 

 like the Sandalwoods (Santalum) ; most are shrubs, like 

 the Mediterranean Osyris alba, or herbs like Thesium. 

 It has been shown that Santalum album can thrive 

 independently, though it is normally a root parasite. A 

 good many santalaceous plants are normally independent ; 

 most, however, require the assistance of a host. There is 

 no high degree of specialisation ; several different species 

 may serve as hosts. 



Germination occurs normally, and suckers are developed 

 first on the side roots ; they resemble those of the 

 Rhinanthoideae, partially surrounding the host root and then 

 sending in an absorbing process in which strands of tissues 

 make contact with the wood of the host (Fig. 24). The 

 leaves show a tendency to reduction in size, perhaps not 

 more than those of other xerophytes ; the plants seem able 

 to assimilate normally. It is again likely that the chief 

 demand on the host is for water and salts. Two genera, 

 Phacellaria and Henslowia, occur as bushes parasitic on the 

 branches of trees in eastern Asia. Their mode of attachment 

 and relations are not known. 



Loranthaceae. — In the Loranthaceae this is the common 

 habit. This great family is predominantly tropical ; it 

 spreads out into the sub-tropics of all the continents, but 



