FLOWERING PARASITES 187 
to germinate unless they happen to lie in 
close proximity to the host of a plant which 
they can successfully attack. his striking 
peculiarity enables us to appreciate some- 
thing of the remarkable qualities which 
render so specialised a parasitic habit feasible 
at all. For the parasite has evidently become 
sensitive to the presence of a definite sub- 
stance which emanates from the host-root. 
The seed is then stimulated, and it awakes 
from its dormant condition. It germinates 
and its roots immediately grow towards, 
and penetrate, the plant from which it will 
ultimately draw practically the whole of its 
food. 
A further state of simplification of vege- 
tative structure is exhibited by Rafflesia 
Arnoldii, which is in many respects perhaps the 
most wonderful of all living flowering plants. 
It occurs in the Eastern tropics, and it pro- 
duces the largest flower known, for it may 
attain to as much as a yard in diameter. 
The rafflesias are mostly parasitic on vine- 
like climbers (Cissus), and they pass their 
vegetative life entirely within the com- 
paratively slender stems of their hosts. In 
this stage Rafflesia is extremely simple in 
structure, and indeed it resembles colourless 
fungal hyphe more than anything else. 
The filamentous cells branch through the tis- 
sues of their host, and it is only when the 
period of flowering draws near that the para- 
site gives any sign of what is about to issue 
