SAPROPHYTISM AND SYMBIOSIS 



771 



fests deciduous trees also appears to be distinct from the forms that 

 parasitize the conifers. Even plurivorous species of Cuscuta grow very 

 differently on different hosts, the number of forms on which they flower 

 and fruit being much less than the number on which they can develop 

 vegetative organs. 



Haustorial structures. The haustoria of parasitic seed plants are 

 much more complex than are those of the fungi, involving various 

 elements in the cortical and vascular tissues. The simplest haustoria 

 occur in such partial parasites as the Euphrasieae, where contact with 

 the host often seems more or less casual. In the mistletoe, also a partial 



FIG. 1084. -The mistletoe, a partial (water) parasite: A, spiny honey-locust trees 

 (Gleditsia triacanthos), many of whose limbs are infested with mistletoe (Phoradendron 

 flavescens); B, a close view of a single mistletoe plant; Rome, Ind Photographs 

 supplied by LAND. 



parasite, but more specialized, the haustoria are prominent organs which 

 exhibit some structural complexity, their hadrome elements coming 

 in close contact with similar elements in the host. Still more specialized 

 are the haustoria of Cuscuta (fig. 1082) and of Orobanche, their terminal 

 cells deploying in all directions, and coming into such close contact 

 with the host cells that sometimes it is difficult to distinguish one from 

 the other, though the frontier cells of the parasite commonly are richer 

 in starch than are the adjoining cells of the host; particularly in Oro- 

 banche are the cortex, leptome, and hadrome of the haustorium in con- 

 tact, respectively, with the cortex, leptome, and hadrome of the host. 

 Perhaps the acme of specialization is seen in Pilostyles, where the 

 haustorial elements permeate the host tissues like fungal hyphae. 



