No. 575] RESPONSES OF PLANTS 



651 



(d) Responses of Sessile Organisms 

 i. Structural Responses. — The striking phases of re- 

 sponses among colonial sessile organisms are often 

 changes in form and structure, or the relative position 

 of the parts. The changes in structure or position of 

 parts are not necessarily advantageous or useful, but are 

 usually so when the stimuli are those commonly encoun- 

 tered in nature (Cowles, '11; Loeb, '06, p. 124; Wood- 

 Jones, '11; Oh. VIII). Indifferent and detrimental re- 

 sponses are o!'t : n giv< n under experimental conditions an 1 

 no doubt the absence of such variants among sessile ani- 

 mals collected in a wild state is due in part to the failure 

 of such organisms to survive. A few sessile colonial 

 organisms such as cacti (Cowles, '11) show little or no 

 plasticity. 



Among sessile animals, the observations of Wood- 

 Jones form the best examples of response. He found 

 that the branching type of corals dominated in barrier 

 pools, tall slender non-branching types in deep water, 

 and massive boulder types on surf beaten shores. Thus 

 he figures similar colonies of each of three genera which, 

 while possessing certain peculiarities of their own, are in 

 general agreement as to growth form just as sessile 

 plants usually are; and this in part for comparable rea- 

 sons. Thus various conifers occur as Krummholz in the 

 high mountains, due to severe conditions (Cowles. '11, 

 p. 732), wind, snow, and in part to the injury of terminal 

 growth regions of the main stem which gives rise to 

 lateral branches. The boulder-like corals with the zooid 

 at the same level occurring on the surf-beaten shores of 

 coral islands are due, in the case of Madrepora, for ex- 

 ample, to repeated injury of the terminal dominant zooids. 

 Conifers in protected situations often grow into tall 



