128 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



tion of the latter is 234 days, and it is possible to recognize four aspects, early- 

 spring, spring, summer, and fall. As the season grows shorter, the early 

 spring and fall aspects merge into the spring and summer respectively. These 

 two aspects persist even when the season is reduced to two months or less, as 

 is the case on Pike's Peak (Clements, 1904 : 349). 



The early spring or prevernal aspect is largely a matter of temperature and 

 light, the water-content being high and the demands upon it slight. The 

 plants are chiefly low mat and rosette plants, which must bloom early to 

 avoid the overshading produced by later species. The maximum water- 

 content occurs in the spring and evaporation is lowest then also, though 

 pronounced fluctuations are of frequent occurrence. As a general result, the 

 more mesophytic species appear in the spring and the more xerophytic ones 

 in late summer and autumn. The maximum development occurs during the 

 summer aspect in high prairie, and somewhat later in low prairie, the tempera- 

 ture necessary for mature growth playing some part in this. In the case of 

 high prairie the growing season is gradually closed by drought, with Iqw prairie 

 it is usually terminated by frost. With the passing of each aspect, its principal 

 species decrease their activity greatly. Accordingly, while the number of 

 mature plants constantly increases during the summer, the demands for 

 water and light increase less rapidly, and the supply is conserved at the 

 requisite level. It is hardly necessary to point out that there are no sharp 

 distinctions between the various aspects. The one passes gradually into the 

 next, and the change is not perceptible from day to day. If, however, the 

 prairie is visited in early April, late in May, in early July and early September, 

 it will present a wholly different appearance at each time. 



Zones and alternes. — The structure of the prairie during a particular aspect 

 is largely due to alternation and zonation. These are both caused by slight 

 differences in the requirements of the species concerned. This is best illus- 

 trated by corresponding species of the same genus, such as Petalostemon pur- 

 pureus and candidus, Psoralea tenuiflora and argophylla, Solidago missouriensis 

 and rigida, Artemisia frigida and gnaphalodes, Aster multiflorus and sericeus, 

 and Liatris punctata, scariosa, and pycnostachya. In each case the first species 

 is more xeroid than the second, with the consequence that one regularly 

 occurs above the other in more or less zonal arrangement over the rolling 

 prairies. In many areas the zones are obscure or interrupted, and the two 

 species occur in drier and moister areas respectively, or the one will be more 

 abundant in high prairie and the other in low prairie. This relation is con- 

 firmed by their successional behavior in that the xeroid species usually shows 

 a marked tendency to appear just before the climax. Petalostemon and Psor- 

 alea have been studied as to their water relations in the Lincoln prairies, 

 where Psoralea argophylla characterizes the valley plains and lowermost 

 slopes with an average water-content of 25 to 35 per cent, while P. tenuiflora 

 dominates the slopes and broad middle ridges with a water-content of 15 to 

 20 per cent. The two touch, but only occasionally overlap or mingle to any 

 considerable degree. Petalostemon has been more adequately studied. The 

 sharp ecotone between the two species was carefully traced on two opposite 

 slopes in 1901, and the water-content limit between them was found to be 

 13 per cent. In 1917, Loftfield studied the water relations of the two species 

 under control, and succeeded in modifying plants of P. purpureus in high 



