No. 575] RESPONSES OF PLANTS AND ANIMALS 



649 



shall call throughout the "dominant apical zooid," constitutes the grow- 

 ing' point; and this zooid is the parent of the entire colony. 



Various writers make comparable statements or show 

 comparable principles among hydroids (Motz-Kossowska, 

 '08) and Bryozoa (Davenport, '91, et al.) and among 

 plants (Goebel, '00). Of the colony form of the tunicates 

 Herdman ('04, p. 82) says: 



The marked differences in the appearance of the colonics of com] ound 

 Ascidians is largely due to the methods of budding: even in those of 

 stolon type where the budding is practically the same in essential nature, 

 the results may be different in superficial appearance, according as the 

 buds are formed on a short stolon close to the parent body, or from the 

 extremity of the post abdomen or from the long epicardiae tube which 

 may extend for some inches from the ascidiozooid. 



Thus we conclude that the innate causes of different 

 growth-forms (colony forms) of colonial organisms are 

 (a) the mode of division of the zooids or vegetative 

 regions, (b) the ratio of stem elongation to number of 

 zooids or buds produced or uniformity or lack of uni- 

 formity of stem elongation (Wood-Jones, p. 76) closely 

 related to (c) the presence or absence, number, position 

 and region of influence of the dominant grmring regions 

 or dominant zooids, and (d), in some cases, the grand 

 period of growth and the length period of the internodes 

 (Johnson, '11). The innate tendencies are thus reducible 

 to a few principles applicable to both plant* and animals. 



(c) Responses of Motile Organisms 

 i. Movements.— In motile organisms the most striking 

 responses are changes in position brought about by 

 movements usually more or less-random, and which bring 

 the organism into various conditions one of which usually 

 relieves the disturbance. The organism resumes normal 

 activity in conditions which brought the relief (Jennings, 

 *06). These conditions are not necessarily advantageous, 

 but are usually so when the stimuli are those encountered 

 in nature (Mast, '11). Behavior of motile organisms is 

 also modified by repetition of action even in animals as 

 low in the animal series as the Protozoa (Holmes, '11). 



