vii.] ANATOMY OF PARASITES 63 



running up the sucker into the root of the 

 parasite and down to the root of the host. 

 These acquire spiral thickenings, and so become 

 "tracheids" in communication with the spiral 

 vessels in the root of the parasite (a). At the 

 other end the bundle of root-hairs, now modified 

 as absorbing organs, penetrates the cortex of the 

 root of the host-plant, dissolving and consuming 

 it by means of a ferment ; but they are stopped 

 by the woody central cylinder which resists the 

 solvent action of the ferment ; consequently they 

 bend round and invest the central cylinder (a). 



The part of the sucker of cow-wheat which 

 enters the host-plant has thus the epidermal 

 layer for its origin. In the yellow rattle 

 additional root-hairs (i.e., besides those forming 

 the penetrating cone) can invest the root of 

 the host and take part in absorbing nourish- 

 ment from it. These, however, do not become 

 spiral tracheids like the former. Other genera 

 show various slight differences, but the general 

 principle on which the suckers are formed in 

 the tribe Euphrasiece is the same ; that is to 

 say, the sucker is epidermal. From the above 

 brief description it would seem that these leafy 

 parasites supply us with a somewhat primitive 

 type of parasitism : first, in their having green 

 leaves ; and secondly, in that the effect of the 

 irritation is more superficial than in the greatly 

 degraded state of leafless and greenless parasites, 



