THE PLANT RECORD. 



293 



leaves, unlike those of grasses, were probably eaten little if at all by grazing 

 animals, while their habit enabled them to be swept and washed into lakes and 

 ponds in larger quantities. 



Kinds of strates. — From the standpoint of succession, the unit strate is one 

 in which the serai development of the climax formation is indicated in some 

 detail. In such cases, the strate proper may consist of more than one layer, 

 though the layers would rarely have a definite serai relation, such as occurs in 

 a stase. Usually, the life-forms of the consocies, in so far as they are repre- 

 sented, are mingled with those of the climax, and every definite and consider- 

 able layer may well be regarded as a strate. When two or more strates 

 marked by the same flora follow each other, they form a castrate. This term 

 may be used even where the strates are separated by non-fossil layers. The 

 strates which record a change from one climax to another, as in the movement 

 of cUmax zones before and after a shifting of cUmates, constitute a distrate. 

 Costrates and clistrates fall within the same great vegetation era, and the 

 sum total of them forms the eostrate. The latter is the complete series of 

 strates in a particular region during the dominance of either of the three 

 great eral floras. Besides the various kinds of strates, it includes stases, as 

 well as non-fossil layers. Thus it is possible, though hardly necessary at 

 present, to distinguish a Paleophytic, Mesophytic, and Cenophytic eostrate. 

 These may be termed respectively paleostrate, meseostrate, and ceneostrate. 

 Taken together, they constitute the geostrate. 



The stase. — A stase is a definite series of layers formed by the remains of 

 the associes and climax of a sere. It is primarily organic in origin and struc- 

 ture, and any great admixture of inorganic materials is evidence of a period of 

 denudation. Not only are the species of each serai stage preserved, but they 

 are also fixed in their areal and temporal relations, so that the developmental 

 sequence is essentially intact. Stases normally arise in the case of hydroseres, 

 though at present the full serai sequence is preserved only in peat-bogs, in 

 which water or a watery substratum is constantly present. Incomplete stases 

 occur where swamps pass into grassland or woodland, or wherever the accu- 

 mulation of plant remains is stopped by the disappearance of the water or 

 moisture which prevents complete decomposition. Fragmentary stases result 

 when an associes or a climax alone is preserved out of the course of the whole 

 sere. 



In addition to water, wind, gravity, volcanic action, or biotic agents may 

 produce stases, though the latter are then nearly always fragmentary, and 

 often transient. Stases due to wind deposit are characteristic of regions 

 where dunes are advanciug over vegetation, especially scrub or forest. They 

 are necessarily fragmentary, since the vegetation is soon killed, and there is 

 obviously no possibiUty of a developmental sequence. Dune stases are Hke- 

 wise transient, owing to the fact that decomposition takes place rapidly in the 

 porous sand, though they may persist for a much longer time whenever the sand 

 is blown away, leaving the dead trunks of the forest or scrub. In the past, 

 loess has doubtless formed similar stases, though the sole evidence of these 

 at present consists of black layers or lines of organic matter, and a few scat- 

 tering plant fossils. The action of gravity in burying communities in talus 

 heaps is essentially similar to that of wind. The stase which results is even 

 more imperfect and transient (plate 54, a, b). 



