MISTLETOE 225 



partial parasite, assimilating carbon dioxide, probably 

 normally, and drawing from the host only water and salts. 

 Its parasitism is, however, a good deal more advanced than 

 that of Thesium or of Euphrasia. Its seed may germinate 

 on a dead surface, but in these circumstances the seedling 

 survives a few months only. It has a highly specialised 

 absorbing system which gives it no possibihty of independent 

 life in the soil or as an epiphyte. Further, it is much more 

 exigent in the matter of hosts. Mistletoe is to be found on a 

 very large number of conifers and deciduous trees ; but, as 

 Heinricher and Tubeuf (1923) have shown, this appearance 

 of wide choice is deceptive. There are, in fact, three distinct 

 races of Visciim album which are morphologically identical 

 (or practically so), but which require different hosts. The 

 first, which grows on the pine, cannot be transferred — by 

 seed or otherwise — to deciduous trees, nor to any conifer 

 except one or two closely related species of Pinus and to the 

 larch. The second occurs on the silver fir, and can be 

 transferred only to some other species of Abies. The 

 third occurs on such broad-leaved trees as the apple, pear, 

 poplar, oak, and is, in its turn, divided into sub-races with 

 more or less sharply marked preferences. The oak is 

 perhaps less often attacked than any other tree, and this 

 rarity may have enhanced the magical properties of the 

 oak mistletoe in the eyes of the Celtic races. The reasons 

 why a particular tree should alone serve as a host to a 

 particular race of mistletoe are not yet known. We here 

 wish to note this specialisation as marking an increased 

 dependence on the host plant. 



It might be thought that in its relations to its host the 

 mistletoe would resemble the relation of a rose grafted on a 

 briar. The rose assimilates carbon dioxide and draws its 

 salts and water from the stock. So intimate is the union 

 that the stock, deprived of foliage, is supplied by the scion 

 with organic food and kept by it in healthy life. The 

 parasite gives no such return. Molisch (1921) has demon- 

 strated this by an interesting experiment. Small apple-trees 

 had several mistletoe seeds sown on them. When these had 



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