200 VEGETABLE PHYSIOLOGY 



themselves upon the roots of the host plant, they are 

 frequently spoken of as root-parasites. From their general 

 structure and their relationship to the host plant, they 

 evidently have much in common with the Mistletoe, and 

 it is not very easy to distinguish between their semi- 

 parasitism and the symbiosis of the latter with the trees 

 on which it lives. They are, however, usually herbaceous 



FIG. 99. Thesium alpinum, SHOWING THE SUCKEBS ON THE HOOTS. 

 (After Kerner.) 



forms, and can therefore be of no use to the host plant 

 in the winter. Moreover, most of them ultimately destroy 

 the root on which they have fastened. 



These root-parasites are mainly members of the Scroph- 

 ulariacece or the Santalacece. As a rule, they are 

 herbaceous annuals, though there are some perennial 

 species. They grow from seed with fair rapidity, the root 

 of the seedling attaining a length of an inch in two or 

 three days. Shortly after penetrating the soil, the main 

 root puts out secondary branches, which make their way 

 parallel to the surface. As they grow chiefly in woods or 

 among herbage, they speedily encounter the roots of other 

 plants, and on contact being made between one of these 



