172 



BOTANY: PRINCIPLES AND PROBLEMS 



Parasites. — The great majority of the seed plants are indepen- 

 dent, deriving their sustenance directly from the inorganic 

 environment, and the struggle between them is therefore fair 

 combat, with victory to the most efficient. Some species, 

 however, have abandoned this passive form of competition for 

 active attack upon their neighbors and have developed an ability 



Fig. 91. — A parasite. The American mistletoe {Fhoradcndron), parasitic on 



tree. 



to obtain part or all of their food directly from the tissues of 

 other plants. Such an organism is known as a parasite (Figs. 

 90 and 91) and its victim as its host. The tissues of the host 

 plant are pierced by small, modified roots, the sucking organs or 

 haustoria, which may be developed either from the root or the 

 stem of the parasite. These penetrate to the vascular bundles 

 or storage regions of the host and draw therefrom a supply of 

 manufactured food. Parasites display certain characteristic 

 structural modifications, notably an absence or poor development 



