116 STRUCTURE AND UNITS OP VEGETATION. 



The disappearance of the layers beneath the primary one follows the life-form 

 sequence, but in the reverse order, the shrubby layer disappearing first, the 

 bushes next, then the tall herbs, and last of all the ground herbs, the mosses 

 and lichens remaining as the final remnant of the layered condition (plate 

 34 b). 



Relation of seasonal aspects- — In forest and thicket, aspects are due to the 

 occurrence of societies at times when light conditions are most favorable. 

 The prevernal aspect of deciduous woods is characterized by a ground-layer 

 of species which develop before the woody plants unfold their leaves and 

 before the other layers have appeared. In general, the herbaceous societies 

 bloom and give character to the different layers in the order of height, so that 

 the seasonal development recapitulates in some degree the succession of life- 

 forms. The seasonal aspects of the prairie show a somewhat similar relation, 

 though the cause is found in the water and heat as well as the light relation. 

 The prevernal and vernal societies and clans are composed of low-growing 

 herbs, such as Anemone, Astragalus, Lomatium, Viola, etc., which correspond 

 to a ground-layer. The summer societies are tall-growing, and often allow 

 the development of one or two layers beneath them. The serotinal aspect 

 is likewise characterized by societies of tall plants, with at least partial secon- 

 dary layers. Apart from the relation of the prairie aspects as layers, there is 

 also a general developmental relation in that the conditions are nearest like 

 those of meadow in the spring, and are most typical of the prairie in summer 

 and autumn. 



THE UNITS OF VEGETATION. 

 HISTORICAL SUMMARY. 



The formation concept. — Although the detailed consideration of the struc- 

 ture of vegetation is reserved for another volume, it is desirable to consider 

 here the chief concepts of the formation. No term has had a more varied 

 experience or a larger variety of uses. Efforts to discard it have been futile, 

 and attempts to definitize it of little avail. Like all plant structures, it is 

 the outcome of development, and hence can not be absolutely delimited. The 

 difficulties in its definition and use seem to have arisen from a failure to 

 recognize its developmental character, as is shown later. As is true of all 

 biological concepts, its first significance was necessarily superficial and incom- 

 plete. But the concept has broadened and deepened until, with the adoption 

 of the developmental idea, it includes the whole group of relations between 

 the basic unit of vegetation and its habitat.. The history of the formation 

 concept is the history of this process of refinement and definitizing. 



Grisebach's concept of the formation.— As is generally known, Grisebach 

 (1838 : 160) was the first to use the word "formation": 



"The first method, the employment of which even a very superficial knowl- 

 edge of a region makes possible, is based upon the physiognomy of vegetation, 

 upon the grouping of individuals in the mass. I would term a group of plants 

 which bears a definite physiognomic character, such as a meadow, a forest, 

 etc., a phytpgeographic formation. The latter may be characterized by a 

 smgle social species, by a complex of dominant species belonging to one 

 family, or, finally, it may show an aggregate of species, which, though of vari- 

 ous taxonomic character, have a common peculiarity; thus, the alpine meadow 



