66 PLANT PHYSIOLOGY 



meristematic tissues (cambium) of the two plants together, with 

 the result that the two stems grow together and form one plant. 

 The root of the stock secures water and mineral salts for the entire 

 plant, while the leaves of the scion carry on photosynthesis for 

 both scion and stock. This seems to be as near a case of genuine 

 mutualism, where both plants profit equally, as can be found 

 among the Spermatophytes. 



Parasitism. — Generally one plant of the union benefits more 

 than the other. If the parasite contains no chlorophyll and gets 

 all of its food from the host (an ironic term) or plant upon which 

 it grows, it is said to be a complete parasite. Among the complete 

 parasites of the seed plants are the dodder (Cuscuta) which grows 

 abundantly on clover, nettles, buckwheat, and many other herbs. 

 The young seedling obtains enough nourishment from the seed to 

 grow until the plant is able to attach itself to a neighboring host, 

 when the roots die severing the connection with the ground; and 

 from then on both food and water are obtained through the 

 haustoria or " suckers" sent into the host. Orobanche, which grows 

 on ragweed, Epifagus (beechdrops) on beech roots, and Conoph- 

 olis (cancer-root) on the roots of oak are other fairly common 

 examples of complete parasites. Rafflesia of Java grows on the 

 roots of trees and sends to the surface only the flower, which, 

 mirabile dictu, is the largest flower known, — over three feet in 

 diameter. 



The mistletoe, which covers the branches of mesquite, poplars, 

 oaks, and other trees in the southern and southwestern United 

 States as well as in various parts of Europe, bears fleshy leaves 

 with a pale green color indicating the presence of some chlorophyll. 

 To what degree this plant is independent of the host for its food 

 supply is not certainly known, but it cannot be a complete para- 

 site. Kostychev showed that in some cases the mistletoe may 

 carry on photosynthesis as actively as autotrophic plants, in- 

 dicating that only water and salts are obtained from the host. 

 And Weir (1916) found that the dwarf mistletoe might even 

 manufacture some food for the plants on which it was growing. 

 Six lodgepole pines on which this plant occurred and (at the same 

 time) six healthy noninfested trees were defoliated. Two years 

 later, those with no mistletoe were dead, while the infested trees 

 were still alive! The pale green color of the leaves and their paucity 

 in chlorophyll, however, indicate that the mistletoe is normally 



