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 
ff Nell 4 
mn 
= SSS 
Fic. 99.—Thesium alpinum, sHowinc THE SUCKERS ON THE Roots. 
(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- 
ulariacee or the Santalacee. 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 
