772 ECOLOGY 



The role of haustoria. The haustoria of parasitic seed plants re- 

 semble those of fungi in the method of penetration. In the mistletoe 

 the pressure exerted by the developing haustorium results in the pene- 

 tration of weak spots, such as lenticels, bark rifts, and cellulose walls; 

 some holoparasites (as Orobanche and Cuscuta) also secrete substances 

 that dissolve the cell walls of the host. In Lathraea the outer haus- 

 torial layer is glandular, secreting a cement, which enables the parasite 

 to adhere to the host. 



The actual substances absorbed by haustoria are inadequately known. 

 In Cuscuta careful cultures indicate that glucose is the chief substance 

 taken from the host, but since the sieve tubes of the host and of the para- 

 site are in contact both in Cuscuta and in Orobanche, it generally has 

 been assumed that proteins are absorbed also. The haustoria of such 

 holoparasites secrete diastases (such as amylase and cellulase), which 

 digest the starch and similar foods of the host. In the mistletoe it is 

 obvious that foods or food materials are taken from the host tree. Be- 

 cause of the hadrome contact and the green leaves, the usual assump- 

 tion is that food materials (i.e. water and salts) are the chief things 

 absorbed; hence such plants have been called water parasites. 



In spite of the possibility that the chlorophyll may not play its usual role, this 

 assumption probably is correct, inasmuch as cultures in weak light result in im- 

 poverished individuals. However, the obvious harm caused to trees by the mistle- 

 toe, the fact that it does not grow on dead trees, and the fact that some mistletoes 

 (as Arceuthobium) are essentially holoparasitic, give some support to the view that 

 the green mistletoes may get certain foods parasitically. 



Of special interest are the Euphrasieae, since, in addition to haus- 

 toria and leaf chlorophyll, there are root hairs which are attached to 

 soil particles. Careful experiments have shown that in this group there 

 exists every gradation from autophytism to holoparasitism. Nearly 

 all species have chlorophyll and all have haustoria, thus appearing to 

 indicate a double nutritive potentiality. At one end of the series is 

 Odontites verna, which can pass its entire life cycle as an autophyte, 

 producing seeds capable of germination; and almost at the other end 

 is Tozzia, a plant without root hairs which requires root contact for 

 germination, and which lives nearly two years as a subterranean holo- 

 parasite before sending up a green aerial shoot that lives but a few weeks. 

 At the extreme end of the series is the holoparasite, Lathraea. The 

 more autophytic species have abundant root hairs and few haustoria, 



