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 

 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 Latlircea and 

 Orobanche, which are members of the British Flora. La- 

 thraea 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 



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 Eafflesiacece. 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, 



