VII. STRUCTURE AND UNITS OF VEGETATION. 

 DEVELOPMENT AND STRUCTURE. 



Relation. — ^Development is the process by which structures are fashioned. 

 This is as true of the climax formation as it is of the mature individual. Each 

 is a climax stage with characteristic structure produced by development. 

 Moreover, both formation and plant exhibit structures in the course of growth. 

 Some of these are retained and contribute to the final form, others are tran- 

 sient and disappear completely after they have fulfilled their function. In 

 the case of the individual, most of the structures persist and play their part 

 in the work of the adult. That this is not necessarily true is shown by the 

 usual behavior of cotyledons and stipules. It is also seen in the complete or 

 partial disappearance of leaves and stems, and especially in the fate of flower 

 parts. From the nature of the plant community, the earlier structures are 

 replaced by later ones, though they may persist in some measure, especially 

 in secondary seres. Finally, the development of both formation and plant 

 is a series of responses to the progressive change of basic factors, which not 

 only control the course of development but determine also its culmination 

 in the adult. 



Kinds of structure. — The nature of succession as a sequence of communities 

 from extreme to medimn conditions determines that its major and universal 

 expression in structure will be zonation. This is convincingly shown in water 

 seres, where the zonation from the center to the margin, due to water rela- 

 tions, is repeated in the zones or layers which succeed each other as the center 

 is shallowed. In essence, the zones of the margin move successively over the 

 surface, and are recorded as superimposed zones in the peat. Whenever 

 conditions change abruptly instead of gradually, zonation is replaced or 

 obscvu-ed by alternation. The latter is strikingly evident in extensive com- 

 munities which are disturbed here and there by denuding agents. The result- 

 ing bare areas give rise to secondary seres, the stages of which when viewed as 

 static communities seem to be unrelated to the circumjacent vegetation. As 

 a matter of fact, they are merely incomplete expressions of successional zones, 

 as is readily observed when the denuding force has operated unevenly over 

 the entire area. The layers of forest and grassland are zonal structures which 

 are more or less evidently connected with succession. The seasonal aspects 

 of vegetation, though recurrent, are also developmental, and often stand in 

 intimate relation to layering. 



Zonation. — Zonation is the epitome of succession. Zones are due to the 

 gradual increase or decrease in a basic factor, typically water, from an area of 

 deficiency or excess. Successional stages are produced by the slow change of 

 a bare area from one of deficiency, e. g., rock, or one of excess, water, to more 

 or less mediiun conditions. In the case of water, for example, the bare area 

 of excess is the starting-point for the series of zones, as it is for the series of 

 stages. In short, zones are stages. This fact has been generally understood 

 in the case of zones around water bodies, in connection with which it was first 

 clearly stated by De Luc (1810 : 140) in the following sentence: 



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