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THE AMERICAN NATURALIST [ Vol. X LV] 1 1 



Wood-Jones finds further that still-water corals are 

 less strongly calcified than those in rough water, the 

 strains producing increased secretion analogous to in- 

 creased tissue production as a result of mechanical 

 strains in plants (Cowles, p. 669). Corals show different 

 kinds of growth under different environments partic- 

 ularly when injured. The new part may be different 

 from the rest and adjusted to the environment thus 

 making it appear as though two "species" occurred in 

 the same colony. The mode of division of the zooid is 

 also different under different conditions. Plants show 

 similar variation with changes of conditions, particularly 

 in the leaves which are divided in submerged portions of 

 amphibious plants and entire in the emerging portions 

 (Cowles, '11, p. 595). 



As has been noted, there is nothing in sessile animals 

 that is more than roughly analogous to leaves. Leaves 

 show marked structural differences on different parts of 

 the same tree where the environmental conditions are 

 different, as, for example, in the differences which occur 

 between the upper and lower portions of a forest tree. 

 While there are, no doubt, differences in similar details 

 (histology) in the organs of display in different parts 

 of the same colony of sessile animals, little or nothing 

 has been done upon them. As a further indication of the 

 prevalence of structural response in sessile organisms 

 of the hydroids Hickson states that there is probably but 

 one species of Millepora which occurs in a large number 

 of growth forms. The commercial sponges (Moore, '08) 

 and common freshwater sponges and polyzoa show many 

 different forms under different environmental conditions. 



The major differences in growth form induced by ex- 

 ternal stimuli in colonial organisms result from modifica- 

 tions of the rate and character of growtli with respect to 

 the fi 



enumerated as follows: (a) mode of division, (fe) amount 



