216 



VEGETABLE PHYSIOLOGY 



While the root is setting up this relationship with a 

 host plant, the shoot of the seedling is growing normally. 

 Its leaves and other sub-aerial parts are well developed 

 and discharge their appropriate functions. The plants 

 would not be recognised at all as in any way parasitic 

 without an examination of the subterranean parts. They 

 absorb 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. In- 

 deed, many of these 

 root-parasites do so 

 little harm to the latter 

 that an affected host 

 is often not noticeably 

 different in appear- 

 ance from a neigh- 

 bouring 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 

 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 



FIG. 104. Thesium alpinum. PIECE OF A 



BOOT WITH SUCKER IN SECTION. X 35. 



(After Kerner.) 



