202 VEGETABLE PHYSIOLOGY 
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 has ceased to act 
agan 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 
Orobanche, which are members of the British Flora. La- 
threa 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 Lathrea possesses no 
chlorophyll. 
Orobanche resembles Lathrea in exhibiting the same 
degree of parasitism. It shows certain differences 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 them- 
selves to their host plant. They constitute the natural 
order Rafflesiacee. The embryo, after emerging from the 
seed, penetrates the cortex of its host, usually a root, though 
not always, and gradually forms a hollow cylinder surround- 
ing its woody centre. This sheathing structure is com- 
posed of rows of cells, and in appearance resembles the 
mycelium of a fungus. Buds arise upon this investment, 
