208 VEGETABLE PHYSIOLOGY 
certain nutritive materials from the roots on which they 
fix themselves, and generally destroy them. The damage 
is, however, local, and does not involve the death of the 
host plant. Indeed, many of these root-parasites do so 
little harm to the latter that an affected host is often not 
noticeably different in appearance from a neighbouring 
plant of the same species which 1g not attacked. 
The ‘perennial forms produce fewer suckers or haustoria 
which only function for one year. The rootlets usually 
bear only one sucker each, and when it hag ceased to act 
as an absorbing organ it dies. The rootlet grows on, and 
in the next year develops a new sucker, and makes a fresh 
attachment. 
Some of these root-parasites are also saprophytic in their 
habit, bearing, besides the suckers, absorbing hairs on 
their underground stems, which come into relationship 
with the humus of the soil. 
There are many other plants which are parasitic upon 
roots, but they must be distinguished from those we have 
just discussed, on account of the greater degree of their 
parasitism. They include such forms as Lathrea and Oro- 
banche, which are members of the British Flora. Lathrea 
obtains food by becoming parasitic on the roots of trees, 
to which its roots attach themselves by suckers, much in 
the same way as the semi-parasites already described. The 
host plant in this case is drawn upon for carbohydrates 
as well as proteins, as Lathrza possesses no chlorophyll. 
Orobanche resembles Lathrea in exhibiting the same 
degree of parasitism. It shows certain ditferences of struc- 
ture, and it does not attach itself exactly in the same way. 
It derives its nutriment entirely from its host, which is fre- 
quently a herbaceous plant. The different species of the 
genus infest different plants, each having only one suitable 
host. 
Some curious parasites which are met with in the tropics 
show a very peculiar method of attaching themselves to 
their host plant. ‘hey constitute the natural order 
