LIFE-HISTORY STAGES. 101 



forms, etc., but also the habitat forms, and somethmg of the reproduction 

 form as well. The life-form, in short, comprises all of the structures which 

 mark the species as an ecological agent. Its fimdamental correspondence 

 with the habitat is obvious. The forms of the aerial shoot are of the first 

 importance, but the organs of perennation have to do directly with occupation 

 and with ecesis. The root-forms are usually of secondary importance, though 

 in sand and gravel in particular they play a conspicuous r61e. In essence, the 

 life-form is the superposition of water and light adaptations upon the vegeta- 

 tion form, though in cryptogams especially, the latter corresponds closely to 

 the reproduction or taxonomic form. 



It is difficult to refrain from speaking of life-forms as lower and higher with 

 respect to their position in succession. This is determined by their demands 

 upon the habitat, as well as by their reaction. In the case of the pioneers of 

 most primary seres this is warranted by the taxonomic development as well, 

 and there can be little objection to this as a convenient comparison. Because 

 of their universal presence, the plankton algse of water-bodies are hardly to be 

 regarded as pioneers in a particular water sere, though this is their position 

 in the geosere. The actual pioneers of a water sere are charads, submerged 

 mosses, and flowering plants, with a life-form characteristic of the habitat. 

 Probably submerged attached algae belong here also. Floating forms, pri- 

 marily phanerogams, mark the first division of the habitat into two media, 

 water and air, and serve as a natural transition to the reed form. In this 

 there is a complete differentiation by the two media into aerial shoot and 

 aquatic roots and shoot. In many cases it is desirable to distinguish the sedge 

 form from the reed, though there is manifestly no sharp line between them. 

 This is true of the grass form in some measure, but it is clear that the habitat 

 has changed materially as a rule. The change from grassland to woodland 

 is the most significant, since the persistence of the stems greatly emphasizes 

 the reaction upon light and other air factors. While the woody form is conse- 

 quently sharply distinguished, this is not always true of the subordinate 

 forms, bushes, shrubs, and trees, since the difference is primarily one of size. 

 In spite of its aerial position, Sphagnum is essentially a submerged moss. 

 In many cases it is clearly a pioneer life-form, though its ability to bring about 

 the swamping of vegetation complicates its treatment. The shrubs charac- 

 teristic of heath belong to a peculiar habitat modification of the shrub form, 

 produced directly or indirectly by acid soil, by deficient aeration or by winter. 



In rock seres, the pioneer life-form is the alga, when the rocks are wet, and 

 the lichen when they are dry. It is interesting, if not significant in this con- 

 nection, that the alga is an essential part of the lichen pioneer. In fact, it 

 seems probable that algae, especially Pleurococcus, may become established on 

 exposed rocks during wet periods and thus actually precede the lichens. 

 Such must be the case with rock lichens in which the spores are still efficient. 

 On moist rocks algae may also be followed by Uchens, especially Collemaceae, 

 though the algal character of moss protonema enables the mosses to appear 

 quickly, and often, it would seem, must enable them to be the first pioneers. 

 On dry rocks there is a fairly distinct successional difference between the crus- 

 tose and fohose lichen forms. The moss form, with its minute rhizoids and 

 power of withstanding desiccation, quickly follows the lichen stages and may 

 even precede the fohose Hchens. The pioneer herb form on exposed rock has 



