NATURAL HISTORY SOCIETY OF GLASGOW. 209 



another source available to them for the supply of nitrogenous 

 nutrition. 



The Mistletoe (Viscum album) is the only woody parasite 

 indigenous in Britain. Loranthus europaeus, another representative 

 of the same order, is found on the Continent, while many allied 

 species are indigenous in the hotter parts of Asia and America ; 

 and all of them are parasitic. They possess no medullary sheath 

 of spiral vessels, but the wood contains scalariform tubes. Their 

 nearest allies are the Santalaceae, and to this order the British 

 plant, Thesium linophyllum, which is recognised as a partial 

 parasite, belongs. The seed of the Mistletoe clings to its host by 

 means of a viscid pulp, and when the radicle sprouts it drives its 

 way through the bark till it reaches the cambium layer, where it 

 connects itself organically. This natural process is in fact similar 

 to the artificial one of grafting, and no further development of 

 root-structure occurs, the full grown plant appearing rootless, like 

 a graft or branch on its host. The Mistletoe is found on a great 

 variety of trees — usually in England, however, on the apple, 

 poplar, or thorn, while its occurrence on the oak is very uncommon. 



The order Orobanchaceae is represented in Britain by various 

 species of Orobanche, and by Lathraea squamaria. These all 

 germinate in the soil, and afterwards attach themselves to the roots 

 of certain plants. The order is probably a parasitical branch of 

 the Scrophidariaceae, in which there are many of the so-called 

 partial parasites formerly referred to, while its exotic genus Striga 

 is distinctly parasitic, and Buchnera hydrabadensis has, like 

 Orobanche, scales instead of true leaves. 



Twining or trailing herbs are, more or less, on the way towards 

 becoming parasitic, and it is not therefore surprising that the 

 Convohidaceae present, as in Cuscuta, instances in which the twin- 

 ing habit has become connected with parasitism. All the species of 

 Cuscuta are leafless; they germinate in the ground, and then coil 

 round other plants, at the same time developing sucker-like bodies, 

 known as haustoria, which in this instance penetrate the tissues 

 of their hosts. These parasites subsequently nourish themselves 

 entirely through the haustoria, their lower parts connected with 

 the soil dying away. They have wire-like stems, with minute 

 scales at the nodes, and tufts of small convolvulaceous flowers. 

 In some parts of England they are very destructive in clover and 

 flax fields. 



