206 VEGETABLE PHYSIOLOGY 
dependence of one form, the parasite, upon the other, the 
host. As in the case of the insectivorous plants, there are 
members of this class which are provided with a chlorophyll 
apparatus, and which are therefore indebted to their hosts 
for protein substances only, or perhaps also for certain of their 
ash constituents. As these almost without exception fasten 
themselves upon the roots of the host plant, they are 
frequently spoken of as root-parasites. From their general 
Ita. 99.—Thesium alpinum, sHowINe THE SUCKERS ON THE Roots. 
(After Kerner.) 
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-para- 
sitism and the symbiosis of the latter with the trées on 
which it lives. They are, however, usually herbaceous 
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 Scrophu- 
lariacece or the Santalacee. As a rule, they are herbaceous 
annuals, though there are some perennial species. They 
