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THE AMERICAN NATURALIST [Vol. XLVIII 



garded as analogous to leaves. Each potential bud with 

 its leaf may be compared to a zooid. In comparing plants 

 and animals, roots can perhaps be compared with the 

 holdfast organs of hydroids. In both groups, roots and 

 root-like organs are individuals of a very low order of 

 indir /dualized inn and of a type not well represented 

 among animals. The holdfast organs of animals are not 

 important absorbers of food and water. 



ii. Stems and Other Connecting Organs (Conducting 

 Tissues).— The most striking difference between the in- 

 complete individualed or colonial plants and colonial ani- 

 mals is the presence in the former of specialized stems and 

 highly complex conducting tissues (Cowles, '11; Putter, 

 '11, pp. 361-66). The conduction of food materials from 

 the root to other parts of the plant and from the leaves to 

 the root is a functional necessity not paralleled even in 

 those colonial animals showing the greatest division of 

 labor. In animals stems are relatively undifferentiated 

 and are often made up of living, relatively unspeeialized 

 zooids, as, for example, in many Bryozoa such as Crisis. 

 The tendency to cauliflory in some plants and the ability 

 of cambium to produce shoots and of the stems of most 

 hydroids to produce individuals indicates that such a con- 

 dition may be potentially present in all. In stalked 

 Protozoa the steins are solid, while in most Coelenterates 

 they are tubes, usually simple though sometimes complex, 

 made up by mere elongation and branching of the stock 

 of the simple single forms such as the Hydra. The lumen 

 is usually ciliated and makes possible a transfer of mate- 

 rial which renders practicable such division of labor as 

 occurs in this group (Putter, '11). In the Bryozoa the 

 different zooids have their body cavities joined in the 

 simpler forms merely as a branching lumen of the main 

 wall of the colony ; in others by small openings the more 

 specialized of which are sieve-like plates (Harmer, '01, 

 pp. 471 and 496; Delage and Herouard, '97, Vol. 5, p. 62). 



The connection between the individuals of the tunicate 

 colonies is often very complex, due to the fact that in the 



