SAPROPHYTISM AND SYMBIOSIS 



769 



terial diseases. In some cases " disease- resistance " involves very 

 complicated phenomena; for example, when the olive is infected by 

 Bacterium Oleae, there are developed in the host plant, layers of scleren- 

 chyma and cork that isolate the bacteria, and antibodies are formed 

 which are specifically toxic to the bacteria. In the lack of all obvious 

 determining factors, appeal sometimes has been made to inherent 

 hidden differences between the various members of a given species, but 

 this " explanation " fails to explain. In any case, immunity is not always 

 or even usually due to 

 the failure of spores to 

 germinate, since para- 

 sitic fungi often are 

 found in various early 

 stages of development 

 on plants which are 

 immune to effective 

 parasitism. 



Parasitic seed plants. 

 • — General features. — 

 Parasitic seed plants, 

 while relatively few in 

 number, are of remark- 

 able interest. A familiar 

 holoparasite is the dod- 

 der (Cuscuta), whose 

 yellowish stem twines 

 closely about that of the 

 host plant (fig. 1081), 

 into which many haus- 

 toria penetrate (fig. 1082). Among the important holoparasites are the 

 Orobanchaceae, a family of root parasites, one of the most interesting 

 of which is Orobanche fasciculata, a frequent parasite on Artemisia 

 (fig. 1083) ; in this species there is but a single point of attachment, which 

 may be on a small lateral root at some distance from the main stem axis 

 of the host. Perhaps the extreme of holoparasitism is found in the 

 tropical families, Rafilesiaceae and Balanophoraceae, in many of which 

 the vegetative body of the parasite is entirely within the host, so that the 

 plant is in evidence only when in blossom; the largest known flower, 

 which sometimes is a meter in diameter, belongs to one of these, Raffle- 



FlG. 1082. — A longitudinal section through two 

 haustoria of a dodder (Cuscuta Gronovii) and a cross 

 section of the stem of the host plant which they are 

 penetrating; note that the vascular tract {v) of the para- 

 site with its hadrome comes into contact with the same 

 region {v') in the host; p, central pith of the host; c, 

 cortex of the parasite ; c', cortex of the host ; e, epidermis 

 of the parasite ; e', epidermis of the host ; r, cambium of 

 the host; highly magniEed. 



