766 ECOLOGY 



on Bromus commutatus do not directly infect B. mollis, but if B. hordeaceus is used 

 as a bridge, that is, if spores from the parasite of B. commutatus are used to infect 

 B. hordeaceus, the spores developing on the latter readily infect B. mollis, which 

 otherwise is immune to this particular parasite. Similarly, by the use of certain 

 hosts as bridges, the ordinary host range of various forms of Puccinia graminis may 

 be extended; for example, barley serves as a bridge from oats to rye or to wheat, or 

 from rye or wheat to oats. While Erysiphe graminis is made up of highly specialized 

 races, a related species, Erysiphe Cichoracearum, is a generalized form; spores of 

 this species that develop on one of the Cucurbitaceae readily infect not only other 

 Cucurbitaceae, but even plantains and sunflowers. Interesting tendencies away 

 from specialization are furnished also by certain parasites that have attacked new 

 hosts, when one or the other is introduced from foreign countries. For example, 

 Puccinia Malvacearum which was introduced from Chile to Europe with a culti- 

 vated Althaea, has spread to species of Malva. The aecidial stage of Cronartium 

 ribicola, a parasite of the European Pinus Cembra, has become a destructive para- 

 site on P. Strobus, which was introduced into Europe from America. One of the 

 most remarkable of plurivorous rusts, in view of the generally close relationship 

 of the hosts of any given parasite, is Cronartium asclepiadeum, a common parasite 

 on Vincetoxicum, one of the Asclepiadaceae, which is equally at home on the peony, 

 one of the Ranunculaceae; recently it has become parasitic also on Nemesia versi- 

 color, one of the Scrophulariaceae; the systematic relationship of host plants is of 

 significance to parasites only as it happens to involve similarity in the physical or 

 chemical character of the substratum. 



The factors involved in progressive variability among the parasitic fungi are 

 quite unknown, and doubtless are much more complicated than among saprophytes, 

 where the culture media may be relatively simple, and capable of analysis. How- 

 ever, it is to be believed that the explanation will be along similar lines, and that it 

 need not involve the mysticism which is inherent in the accommodation theory. 



The origin of parasitism in fungi and bacteria. Parasites appear 

 to be more highly specialized than saprophytes, and most parasitic fungi 

 and bacteria probably have arisen from algae by way of saprophytism; 

 among the thallophytes there are many transitions between mixophytes 

 and holosaprophytes, and between the latter and holoparasites, but few, 

 if any, between mixophytes and holoparasites. 1 The probable stages in 

 the origin of saprophytism have been mentioned. The next step, 

 namely, the development of facultative parasitism, seems to follow 

 easily, and is illustrated by many existing species (p. 762). The succeed- 



1 There are a few parasitic algae that may have developed directly from mixophytes 

 into parasites rather than by way of saprophytism. One of the most interesting of these 

 parasitic algae is the destructive tea rust, Ceplialeuros virescens. Another parasitic 

 alga is Rhodochytrium Spilanthidis, which grows on Ambrosia and on other hosts. There 

 are also a number of marine red algae that are parasitic; among these is Choreocolax 

 Polysiphoniae, which is completely colorless and has plasmatic connections with its 

 algal host. 



