ECADS. 67 



greater accuracy and certainty. This will come about from the quantitative 

 study of their physiologic response, permitting the closer correlation of form 

 and function, as well as by the increasing use of standard plants as even more 

 accurate indicators. 



Habitat-forms can be used to give a general statistical expression to the 

 climatic or physiographic conditions of a region, and thus permit comparisons, 

 much as Raunkiaer has used vegetation-forms. Their paramount value lies 

 in their positive indication of definite local conditions on the basis of known 

 correlation with measured factors. It should be noted that the mesophytes 

 and the last three groups of xerophytes represent climax habitats and com- 

 munities, while the hydrophytes and the first six groups of xerophytes charac- 

 terize developmental stages. This is a natural outcome of the fact that the 

 climate is controlling as to soil conditions in the former, while the climatic 

 control is much reduced or is none at all for the latter. The general correla- 

 tion of climax habitat-forms and their most important representatives with 

 physical factors is given in Chapter IV, in so far as quantitative results are 

 available. 



In a recent paper, Raunkiaer (1916 : 225; cf. Fuller and Bakke, 1918 : 25) 

 has sought to express the general relation of plants to climate by a series of 

 leaf classes based upon size. Of the latter, he recognizes six kinds as fol- 

 lows: leptophyll, 25 sq.mm.; nanophyll, 9 X 25 sq. mm.; microphyll, 9 2 X 25 

 sq. mm. ; mesophyll, 9 3 X 25 sq. mm. ; macrophyll, 9 4 X 25 sq. mm. ; megaphyll. 

 While this classification will serve a useful purpose in drawing the attention 

 of ecologists to such relations, it seems quite too subjective for final accep- 

 tance. This seems obvious from the author's difficulties as to compound and 

 lobed leaves, and especially from the following statement (1. c, 29) : 



"Originally I multiplied by 10, but the resulting limits between the 'size- 

 classes' did not seem as natural as when 9 was used. It is easy in the final 

 analyses to separate the single classes into the groups of small, medium, and 

 large." 



Thus, while there can be little question that leaf-size often serves as an 

 indicator of climate or habitat in some degree, it must be refined by means of 

 leaf-number, thickness, structure, outline, and texture, and checked by quan- 

 titative studies of factors (cf. E. S. Clements, 1905 : 91). 



Ecads. — An ecad is produced by direct and demonstrable adaptation to 

 a habitat. It is a habitat-form in the making. The habitat-form, while 

 capable of modification within certain limits, has recorded the impress of a 

 particular habitat for so long that its general character is fixed and trans- 

 mitted. An ecad, though it may show just as striking adaptation, is a recent 

 product, and its character is not yet fixed and transmissible. The difference 

 between the two is solely one of inheritance, and it seems probable that ecads 

 become fixed and pass over into habitat-forms after a long residence in the 

 same habitat. This is indicated by the behavior of alpine dwarfs, some of 

 which retain their form when moved to lower altitudes or shifted to wetter 

 alpine situations, while others at once change in response to the new condi- 

 tions. The former have attained the stability of habitat-forms, the latter are 

 ecads. 



