68 BASES AND CRITERIA. 



Because of its plastic nature, the ecad is a more exact and sensitive indicator 

 than the habitat-form. Its structural change corresponds more nearly to the 

 functional response and can be regarded as a measure of the latter to a con- 

 siderable degree. Its growth as well as its form is often characteristic, and its 

 indicator value can be based upon both. One unique advantage of the ecad 

 is that it is produced in abundance in nature, wherever habitats touch, espe- 

 cially where they recur constantly, as in mountain regions. A plastic species 

 found in two or more habitats regularly shows an ecad corresponding to each. 

 Similar results are readily obtained by transplanting such species to several 

 different habitats. Ecads produced under definite quantities of water and 

 light may be grown under control (Clements, 1905 : 157; 1919) and used for 

 comparison with the natural ones (E. S. Clements, 1905) (plate 11). 



Ecads have been classified and named with reference to habitats, as hylo- 

 colus, psilocolus, etc. (Clements, 1902 : 17; 1904 : 329). It seems much better 

 to group and designate them with reference to the controlling factor (Clements, 

 1908 : 263), as water ecads, light ecads, etc. Thus the general classification 

 of ecads would necessarily correspond closely to that of habitat-forms, except 

 in xerophytes, where the groups would be fewer. Such a classification would 

 be of little value, however, since it is the relationship of the ecad to a particu- 

 lar species which is significant, as well as the number and kind of ecads actually 

 occurring. A floating species, such as Sparganium angustifolium, forms both 

 submerged and amphibious ecads, while Nymphaea polysepala has been seen 

 to produce only amphibious ones. A plastic helophyte, such as Ranunculus 

 sceleratus, or a mesophyte, such as Achillea millefolium, may give rise to 

 several ecads. The same species may produce both water and light ecads, 

 though as a rule wide a range of adaptation to the one factor is accompanied 

 by a narrow range for the other. Under control it has been possible to produce 

 ten distinct water ecads of Ranunculus, but beyond this point differences have 

 to do chiefly with amount of growth rather than with structure. For the 

 present, it is sufficient to recognize the controlling factor by designating ecads 

 as hydrads, xerads, sciads, heliads, halads, etc., and to leave the question of 

 a more exact terminology for the future. The importance of ecads in indicator 

 work is so great that their recognition can no longer be neglected. 



GROWTH-FORMS. 



Nature. — While it is assumed that all plant forms are referable to the 

 immediate or remote action of the habitat, this correlation is least certain for 

 taxonomic forms. Its certainty increases progressively through life-forms 

 and habitat-forms to reach a maximum in growth-forms. While Warming in 

 particular has used this term in place of life-form and vegetation-form, the 

 latter have the preference, both by priority and significance. But growth- 

 form is such a desirable term for the immediate quantitative response made 

 by a plant to different habitats or conditions that its retention in this sense 

 seems well-warranted. As the direct visible response of the plant to physical 

 factors, growth affords a more delicate scale of measurements even than the 

 ecad. In fact, the latter is only a growth-form in which adaptation as shown 

 by a qualitative change of form or structure is more striking than the quanti- 

 tative difference in amount of growth. In the case of dwarfing, both changes 

 usually occur together, and the growth-form differs from the ecad only in 



