No. 575] BESPONSES OF PLANTS AND ANIMALS 655 



(c) The breeding activities are (c) The reproductive organs 

 probably least modifiable and least and early embryonic stages are 

 regulatory. less modifiable than the vegetative 



(a, b, c) The maple tree, a sessile organism, is entirely 

 stationary in its adult stages. The seeds are blown by 

 the wind. One wonld not accomplish much in the study of 

 ecology by studying the distribution of the seeds of the 

 maple, or, on the other hand, by the study of the distribu- 

 tion of adult birds, without some further discrimination. 



Sessile organisms are not difficult to associate with 

 their proper environmental conditions in their adult 

 stages. As we proceed in our study to forms which can 

 move readily and rapidly, the difficulty of associating 

 them with their definite environmental conditions in- 

 creases. Sessile organisms have stages which are small 

 and capable of easy dispersal, as in the case of the maple. 

 Sessile marine animals and some sessile plants frequently 

 have motile forms in young stages. In these motile 

 stages they are governed by the same laws as other motile 

 organisms. The conditions under which the motile stages 

 develop into the sessile forms are crucial. 



Most fresh-water forms and some marine forms of 

 sessile organisms are without the free-swimming stage, 

 and they produce non-motile stages physiologically 

 comparable to the seeds of higher plants. The winter 

 bodies (statoblasts) of the Bryozoan (Pectinatella) com- 

 mon near Chicago, and which is a strictly sessile organ- 

 ism, are comparable to seeds and probably require 

 "ripening" by cold, just as do many seeds and the repro- 

 ductive bodies of some other species of the same group. 

 Organisms which are highly motile in the adult stages 

 are not motile in the egg and young stages. The eggs 

 and young of birds, for example, do not move about, yet 

 birds are the most motile of all animals. 

 2. Comparison of the Sessile and Motile Elements of 

 the Biota 



(a) The motile organisms of a (a) The sessile organisms of a 



