74 I The Process of Evolution 



consisting of both alga and fungus. Often the alga can be grown 

 without the fungus, but the latter does not survive without its algal 

 host. 



Complex colonial organisms also present difficulties. The colonies 

 of social insects present analogies with organisms, but usually such 

 colonies are referred to as quasi organisms. The Portuguese man-of- 

 war, a colonial hydrozoon, can be analyzed into its constituent 

 polyps, which exhibit a striking division of labor. Among the algae 

 and protozoa there are less specialized aggregations of individuals, 

 in which what appear to be units may exist separately or as part of 

 the colony. Even such forms as yeast (Saccharomyces) may show 

 diJBFerent behavior, depending upon environmental conditions. In 

 liquid culture, yeast cells (plants?) are small ovoid cells that re- 

 produce most frequently by budding. Short chains of cells may 

 occur. When grown on a solid medium, however, yeast forms a giant 

 "colony." This structure is a flattened object, several centimeters in 

 diameter, with characteristic color and surface texture as well as bio- 

 chemistry. Cells from the outermost layer, from the center, and from 

 the portion adjacent to the medium are very different in form and 

 presumably in function. Nevertheless, cells from any region may be 

 used to start a new colony or liquid culture. 



In the higher organisms there also may be difficulties in defining 

 individuals. Many plants reproduce vegetatively (see Chap. 9), and 

 if the "offspring" remain attached to the parent, the whole is con- 

 sidered an individual. Should they become separated, each plant 

 usually is thought of as an individual even though it is genetically 

 identical with its "parent." The self-sterile triploid day lily Hemero- 

 callis fulva is one genetic individual throughout its range in much 

 of the eastern half of the United States. Populations of hydra derived 

 from a single budding individual likewise genetically constitute an 

 individual, but ecologically and functionally they consist of many 

 individuals. 



Complexes of individuals belonging to what are called different 

 species may also occur. Many scale insects form amazing compound 

 colonies in symbiotic association with a fungus (Septobasidium). 

 Forest trees commonly become grafted when their roots touch in the 

 course of growth. It has been found that, if a root-grafted tree is cut 

 down, the stump may survive for many years. Although without 

 photosynthetic tissue of its own, it may produce new bark from the 

 cambium so that the stump is completely covered. Individual organ- 

 isms are genetically different in these situations, but they are united 

 closely into an ecologically meaningful unit. In the same way, a 

 clone of viviparous onions (see Chap. 9) that are genetically iden- 



