GROWTH-FORMS. 69 



being the product of the conditions presented by a single season. If these 

 continue, the growth-form persists and becomes an ecad characteristic of the 

 particular habitat. Thus, while the two forms may be measures of the same 

 conditions, the one is an indicator of the annual variation, the other of the 

 normal condition of the habitat. From the ecological side, it appears that 

 growth-forms may become ecads, ecads become habitat-forms, and these 

 finally fixed as vegetation-forms. 



Kinds. — Every direct factor exerts an influence upon growth and produces 

 corresponding growth-forms. Such factors are water, fight, temperature, and 

 aeration, and possibly certain solutes. Since all of these are concerned in the 

 growth of each plant, it is possible to assign a particular one as the cause of 

 any growth-form only when it is the controlling or limiting factor. In the 

 majority of cases, the limiting action is evident, as with water in arid and 

 semi-arid habitats or dry seasons, light in forests and thicket, temperature in 

 high altitudes or latitudes or cold seasons, and aeration in wet areas or seasons. 

 Maximum growth results when all four factors are at the optimum for a par- 

 ticular species. An apparent exception is afforded by the behavior of many 

 species in moderate shade, but their height is usually offset by their slender- 

 ness, and the mass growth and dry weight are usually less than in the sun. 

 With the optimum growth as the basis, it becomes possible to distinguish 

 growth-forms due to the extremes of each factor, as well as to correlate differ- 

 ent amounts of growth with known quantities of the limiting factor. In the 

 case of water, growth is decreased by both an excess and deficit as a rule, but 

 the former seems to operate through reduced aeration and lowered tempera- 

 ture. Similarly, growth is diminished by both high and low temperatures, 

 but high temperatures act chiefly through the water relation. It is doubtful 

 whether full sunshine as light ever inhibits growth, since photosynthetic 

 activity decreases with any material reduction in light intensity. While 

 many species are taller and more branched in moderate shade, it appears that 

 mass growth is at a minimum and often becomes completely impossible with 

 the increasing density of forest or thicket. 



As a consequence of the above, it is most practical to distinguish four types 

 of growth-forms, based upon the lack of the direct limiting factors, namely, 

 those due to insufficient water, to insufficient heat, to shade, and to poor 

 aeration. Since growth is primarily quantitative, each species will exhibit a 

 series of forms from the optimum to the minimum, corresponding to each 

 effective degree of change in the limiting factor. This relation lies at the base 

 of ecological response and can only be determined experimentally. Two 

 factors may act together in producing a growth-form, as in the case of alpine 

 dwarfs due to drouth and low temperature. One factor may serve to empha- 

 size another, as where the drouth of a desert is reinforced by an excess of salts 

 in the soil, or it may decrease or counteract the effect of another, as is true of 

 shade in arid regions. Finally, all four factors may be concerned causally in 

 an effect produced directly by one of them. This is apparently the case in the 

 death of sal seedlings in tropical forests,as shown by Hole and Singh (Chap. III). 

 The immediate cause is poor aeration, due to the accumulation of soil-water as 

 a consequence of lower temperature resulting from shade. 



Indicator relations. — The growth of a species varies from one year to the 

 next, and from one habitat to another. It often differs also in different por- 



