INFLUENCE OF ENVIEONMENT ON PLANTS 343 



similar degradation of structure. They possess no chloro- 

 plasts, their leaves are absent or reduced to the condition 

 of scales, while their stems are often thick and succulent. 

 Their roots are replaced by the so-called haustoria, which 

 penetrate into the tissues of their hosts, complete fusion of 

 the tissue of the host and the parasite frequently taking 

 place. We have representatives of such parasites in the 

 British flora in the Cuscutas and the Orobancliacece. 



Many of the plants belonging to the Santalacece and 

 the Scrophulariacece show a partial parasitism of this 



FIG. 148. Thesium alpinurn. PIECE OF A 



BOOT WITH SUCKER IN SECTION. X 35. 



(After Kerner.) 



kind. They have short stems which bear green functional 

 leaves, but are peculiar in that their roots become attached 

 by curious sucker-like bodies to the roots of other plants 

 growing near them (figs. 147, 148), and from these suckers 

 absorbing cells are developed which penetrate into the 

 substance of their hosts and draw nourishment from them- 

 They are generally described as root parasites. The 

 Mistletoe behaves similarly, striking its haustoria into the 

 tissue of the branches of the apple, oak, poplar, &c. The 

 parasitism is partly compensated by the fact that its leaves 



