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 is 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 has 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 then- 

 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 Lathrcea and Oro- 

 banche, which are members of the British Flora. Lathrsea 

 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 Lathraea possesses no chlorophyll. 



Orobanche resembles Lathrsea 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 themselves to 

 their host plant. They constitute the natural order 



