plants arc, of course, sessile ; most animals, motile. 

 There are, however, some motile ])lants among uni- 

 cellular forms and male gametes, and there are many 

 sessile or slow-moving animals in a(|uatic habitats. 

 Sessile organisms respond to variations of the en- 

 vironment primarily by changes in form ; motile ani- 

 mals. ])riniarily by changes in behavior. 



MOKIMIOI.OGICAL .AD.APT.^TIONS 



('hangcs in form and siriirliiro 



is a similar moriihological res|)onse to a similar en- 

 vironment (Schmaulhausen 1949). If the growth- 

 form ])ersists through many generations and appears 

 to he an adajjtation. even though not inherited, it is 

 often called an ccad. If and when the growth-form 

 becomes inherited as the result of evolutionary proc- 

 esses, it then becomes an fiolyf<c. Lije-form is a gen- 

 era! term referring to the sha|)e or appearance of an 

 organism irrespective of how formed (Daubenmire 

 1947 ). The prevalence of particular life-forms among 

 the im])ortant organisms helps to separate and char- 

 acterize biotic communities, as we will see repeatedly 

 in later discussions. 



Consider a sessile organism, the tree. It is essen- 

 tial to the tree that its foliage be exposed to sunlight. 

 As it grows within a forest, it is usually tall and 

 slender, and little branched except at the top. where 

 the cap of foliage reaches into the full sunlight. Grow- 

 ing on the forest's edge, the tree is shorter, and 

 branching and foliage are dense both at the cap and 

 on that side exposed to full sunlight. The tree which 

 grows solitary in an open place is short, but branch- 

 ing and foliage are dense and uniformly distributed, 

 often starting close to the ground. In similar manner, 

 the variations in form assumed by sessile colonial 

 animals, such as sponges and corals, reflect vicissi- 

 tudes imposed by habitat (Wells 1954). 



Morphological variations induced by peculiarities 

 of habitat do occur in motile animals : thickening of 

 the shells of clams subjected to strenuous wave ac- 

 tion ; variation in number of vertebrae, scales, and fin 

 rays among fish subjected to different temperatures 

 at critical periods in their growth (Taning 1952) ; 

 changes in the number of facets in the bar-eye of the 

 fly Drosophila as a correlative of temperature varia- 

 tions during a short critical period in larval growth 

 (Krafka 1920) ; the many variations in form and size 

 of internal parasites, depending on crowding and 

 other environmental conditions (Baer 1931) ; pointed 

 tails in certain flatworms crawling over a substratum 

 during growth, contrasted with rounded tails occur- 

 ring in the same species when individuals are experi- 

 mentally prevented from crawling over the sub- 

 stratum (Child 1903). 



That individuals of the same species are so much 

 alike attests the great extent to which the course and 

 outcome of morphological development is genetically 

 determined. But that there are variations between 

 individuals, and between groups of individuals, of the 

 same species shows that morphological development 

 is also responsive to environmental influences. Modi- 

 fications induced by the environment emerge as the 

 individual develops and are not specifically inherited 

 by the succeeding generation. These modifications 

 are called growth-forms. If the generation following 

 is similar in growth-form to the parent generation, it 



Life-forms of plants 



The life-form of a plant is characterized by its 

 vegetative form, its length of life, the arrangement 

 and character of its leaves, whether its stem is her- 

 baceous or woody, its manner of growth, and its 

 means of overwintering. Life-form categories some- 

 times agree with large taxonomic units, such as ferns 

 and mosses, but on the other hand some taxonomic 

 groups contain s])ecies exhibiting a variety of life- 

 forms and some life-forms include species only re- 

 motely related taxonomically. 



There have been many systems proposed for the 

 classification and terminology of the life-forms of 



FIG. 2 1 Form assumed by the coral Modreporo as If develops 

 In (a) deep water; (b) barrier pools; (c) rough water (from 

 Wood-Jones 1912). 



The general nature of responses 7 



