LORANTHACE^: LATHRiEA 229 



are compared. He concludes that the chief danger to the 

 host is the withholding of water from leaves lying above the 

 point of attachment of the parasite, through the competition 

 for the supply by the latter. He states that leaves in such 

 a position do frequently wither. McLuckie (1923) notes 

 that the hosts of Loranthus celasiroides in New South Wales 

 may be killed by the withdrawal of water. Future work 

 must show if the relation is general. 



Complete Parasites. — The mistletoe may be taken as the 

 most advanced of the semi-parasites, and we may now 

 return to the Rhinanthoideas and trace out another line 

 along which parasitism has evolved. In the genus Tozzia, 

 with one species in the Alps and another in the Carpathians, 

 the habit is somewhat that of a large eyebright or a yellow 

 rattle, but with distinctly fleshy shoots and pale green 

 leaves obviously poor in chlorophyll. The life-history is 

 curious. For two or three years after the germination of 

 the seed the plant exists as a subterranean rhizome with 

 scale leaves, wholly parasitic on the roots of the hosts ; then 

 the leafy aerial shoot is sent above ground, and the plant 

 leads a subaerial and semi-parasitic existence for a few weeks 

 before flowering. 



Finally four genera of the group are complete parasites. 

 Of these the best known is Lathraea with two (European) 

 species, L. Squamaria, the toothwort, which occurs in 

 Britain, and L. clandestina. The toothwort has a thick 

 flowering axis above the ground ; it is purplish in colour, 

 possesses only small scale leaves, and ends in a raceme with 

 numerous flowers. Neither axis nor scales possess chloro- 

 phyll ; the plant is completely parasitic. The inflorescence 

 springs from a thick, branched, subterranean rhizome bear- 

 ing fleshy scale leaves in four rows. The structure of these 

 scales is peculiar. The upper part is reflexed and united 

 to the base, forming a hollow with a narrow opening ; this 

 enclosed space communicates with branching canals 

 extending into the leaf tissues. The inner surface is covered 

 with epidermal glands which excrete water. Various 

 functions have been assigned to these remarkable leaves. 



