342 



VEGETABLE PHYSIOLOGY 



freely by their whole surface. They have, therefore, no need 

 of differentiated absorbing or conducting tissues, which are 

 consequently not developed. A further consequence of the 

 ease with which they obtain their food is the readiness with 

 which vegetative and asexual reproduction is brought about ; 

 hence sexuality is in many cases non-existent among them. 

 Phanerogams which are completely parasitic show a 

 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. 



FIG. 146. Thesium alpimtm. PIECE or A KOOT WITH SUCKER 

 IN SECTION, x 35. (After Kerner.) 



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 Cuscuta and the Orobancliacece. 



Many of the plants belonging to the Santalacece and 

 the ScrophulariacecB show a partial parasitism of this 

 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. 145, 146), and from these suckers 



