786 ECOLOGY 



Fasciation. — Perhaps to be classed with galls are the peculiar structures known 

 as fasciations, which usually are manifested in the form of flattened stems or 

 branches. The form may arise through the flattening of a single cylindrical stem, 

 or a number of stems may be merged into a single fasciated structure. Often the 

 flowers as well as the branches are modified in appearance. The phenomenon is 

 not well understood, but often it is believed to be associated with " over-nutrition "; 

 sometimes it is produced by mechanical causes, or by insect or fungal activities 

 (as in Oenothera). Fasciation sometimes appears to be inheritable, but this re- 

 mains to be established, at least as a general proposition. 



Autoparasitism. — Auloparasitism {i.e. self-parasitism) is a common phenome- 

 non, since most plants contain colorless living tissues that derive their food from 

 the green chlorenchyma. In some cases green plants bear albjscent shoots whose 

 nourishment is parasitic; in the redwood such shoots occur frequently, and when they 

 are detached and planted in the soil, they may develop chlorophyll. In plants pos- 

 sessing an alternation of generations, one phase commonly is parasitic on the other. 

 For example, in liverworts and mosses the sporophyte foot (an organ resembling a 

 haustorium) is embedded in gametophyte tissues, and in Anthocera it has rhizoid- 

 lilce processes (fig. 241); since the moss sporophyte commonly is green, it probably 

 is a water parasite comparable to the mistletoe, though in Sphagnum the sporophyte 

 is colorless and holoparasitic (fig. 250). In the seed plants the gametophyte is 

 parasitic on the sporophyte, and sometimes there are haustorial processes, as in 

 Zatnia ; the embryonic sporophytes also are parasitic, the suspensors frequently 

 resembling haustoria and acting as organs of food absorption and conduction (figs. 

 460, 510). Parasitic features are exhibited by many germinating seedlings, partic- 

 ularly among the monocotyls, in which the tip of the cotyledon often is a haustorial 

 organ (figs. 1229, 1230) and secretes digestive enzyms. Grass seedlings have a 

 peculiar organ, the scutellum, which connects the embryo with the region of accumu- 

 lated foods; often the scutellum cells are elongated, in Briza even resembling root 

 hairs. Perhaps to be noted under autoparasitism is the occasional parasitism of a 

 dodder plant upon another dodder, of mistletoe upon mistletoe, or of strong individ- 

 uals upon weak individuals among the Euphrasieae. 



3. RECIPROCAL PARASITISM, HELOTISM, AND 

 ENDOSAPROPHYTISM 



Definitions. — The topic symbiosis commonly has been subdivided 

 into antagonistic symbiosis or parasitism and cooperative symbiosis or 

 mutualism. Such terms as mutualism and cooperation are humanistic, 

 and should be discarded. Each of tvifo symbionts may be benefited 

 nutritively or otherwise by the presence of the other, but it is a miscon- 

 ception to regard two symbionts as giving one another food or assistance. 

 That form of symbiosis in which each of the symbionts obtains food 

 from the other may be termed reciprocal parasitism; where the para- 

 sitism of the two symbionts is alternative rather than simultaneous, the 



