PHANEROGAMIC PARASITES 69 



another parasitic plant (Loranthus europaeus). Those trees 

 having a soft cortex with a very tender cork-tissue are most 

 infested, as poplars, willows, apple-trees, etc., the black poplar 

 being an especial favourite. Certain conifers are also 

 attacked, as the silver fir, Corsican pine, and Scots fir, but the 

 mistletoe never appears to be quite happy when growing on 

 conifers, and under those circumstances its seeds never 

 contain more than one embryo, whereas when growing on 

 broad-leaved trees, more especially on poplars, the seeds often 

 contain two or even three embryos. The mistletoe is dis- 

 seminated entirely through the agency of birds, more especially 

 the thrush, which feeds upon the white berries, and deposits 

 the undigested seeds along with a mucilaginous mass upon 

 the branches of trees to which they become fixed by the har- 

 dening of the viscid substance surrounding them. When the 

 mistletoe seed germinates its radicle comes in contact with 

 the bark, where it forms an attachment-disc, from the centre 

 of which a slender prolongation pierces the bark of the host- 

 plant as a specialised kind of root. This root or sucker 

 penetrates until it just reaches the wood of the host-plant. 

 During the following year as the branch of the host increases 

 in thickness, by the formation of another ring of wood, a zone 

 of active cells at the base of the sucker commences growth 

 and adds to the length of the sucker, which elongates out- 

 wards or away from the branch, and thus prevents the 

 mistletoe from being engulfed in the increasing thickness 

 of the branch upon which it is parasitic. By such an arrange- 

 ment, although the sucker has not actually grown deeper into 

 the wood, it elongates upwards each year, and is eventually 

 surrounded by many annual rings of wood. During this 

 period the growing zone at the base of the sucker sends off 

 lateral strands of tissue in every direction just between the 

 bark and the wood, and these in turn give off a series of 

 suckers which develop in a manner similar to the primary one. 

 By this arrangement the parasite continues to draw a constant 

 supply of food from its host-plant. 



The last batch of parasites belong to the Figwort family 

 (Scrophulariaceae), and bear green leaves and resemble super- 

 ficially those plants that derive the whole of their food from 

 the air and from substances dissolved in water. These plants 

 are the cow-wheats (Melampyrio?i), eyebrights {Euphrasia), 

 bartsias (Bartsid), yellow-rattles {Rhi/ianthus), and louse- 

 worts (Pedicularis). After the germination of the seed the 



