774 ECOLOGY 



plants serving equally well. Upon germination peculiar hair tendrils 

 arise, and further development depends upon their coming into contact 

 with the roots of certain monocotyls. 



In some partial parasites (as Melampyrum pratensv and Santaluni) 

 haustoria appear to originate without special contact stimuli, though 

 ' their full development usually requires contact with a living host; for 

 example, in Melampyrum, tracheids become differentiated only after 

 such contact. In holoparasites (as Cuscuta and Lathraea) and probably 

 in most partial parasites (e.g. Odontites), even the first stages of haus- 

 torial development require contact with a living host. That the ques- 

 tion is not merely one of contact, however, is shown by the fact that 

 non-nutritive solid bodies do not stimulate haustoria. It has been 

 claimed that when an Epifagus plant conies in contact with a beech 

 root, haustoria develop on the host rather than on the parasite. Piob- 

 ably the factors involved in haustorial stimulation are chemical in 

 nature and exceedingly complex. 



In the Euphrasieae many of the species exhibit a high degree of variability, the 

 amount and character of which depends upon the conditions to which they are 

 exposed. As previously noted, some of this group (as Lathraea) may be ei her 

 saprophytes or parasites, and others (such as species of Melampyrum and Odont-tes) 

 may exhibit varying degrees of saprophytism, parasitism, and autophytism. \V hen 

 Odontites verna is grown in humus, only 10 per cent of the plants are attached to 

 hosts, and yet the plants are more vigorous than in sand, where 43 per cent exhibit 

 parasitic attachment. Parasitic Euphrasieae are much more luxuriant on vigorous 

 hosts than on weak hosts, developing larger seeds and having larger, healthier, and 

 more autophytic progeny. The progeny of weak parasites is much more likely 

 to be albescent than is the progeny of strong parasites. The latter phenomena are 

 very suggestive from an evolutionary standpoint. The culmination of variability 

 appears to be in Alectorolophus, where many so-called species have been found 

 experimentally to be merely habitat varieties. 



Holoparasites are much more specialized, and are more completely depen lent 

 upon their hosts than are partial parasites; hence they exhibit much less variability. 

 However, Cuscuta shows interesting variations which indicate that probably it has 

 not reached the utmost bounds of parasitism; for example, Cuscuta mono^yna 

 has been grown to maturity as a saprophyte in glucose solutions. While the pres- 

 ence of plastids in Cuscuta has been both affirmed and denied, there is little doubt 

 of the presence of chlorophyll when the dodder is grown in the shade or on starved 

 hosts ; indeed, there is evidence that the dodder is able to manufacture small quan- 

 tities of carbohydrate food. Cassytha, a member of the Lauraceae, is a parasite 

 of similar aspect, and likewise is yellow in the sun and green in the shade. When 

 Cuscuta grows on members of the Solanaceae, the haustoria secrete an oily substance 

 not noticeable elsewhere; since the poisonous alkaloids of the Solanaceae do not 

 enter the parasite, it has been suggested that the haustorial secretions may have 



