342 VEGETABLE PHYSIOLOGY 
treely 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. 
Fic. 146.—Thesium alpinum. Prncr oF a ROOT WITH SUCKER 
IN SECTION. x 85. (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 Cuscwta and the Orobanchacee. 
Many of the plants belonging to the Santalacee and 
the Scrophulariacee 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 
