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 



FIG. 99. Thesium alpinum, SHOWING 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 trees 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 Santalacece. As a rule, they are herbaceous 

 annuals, though there are some perennial species. They 



