DEVELOPMENT AND STRUCTURE. 115 



Significance of alternation. — ^Alternation is the consequence of disturbed or 

 incomplete zonation. Such areas produce alternes, which it now seems can 

 always be related to more primary zones. This has already been shown in 

 the case of the alternes of canons, which are only the upward or downward 

 extension of zones. Wherever the conditions which control zonation are dis- 

 turbed, alternation is produced, just as is the case whenever the conditions 

 for a particular zone occur abruptly or locally. An excellent example of the 

 latter are the extra-regional pockets of Celiis or Symphoricarpus, described by 

 Pool (1914) in the sand-hills of Nebraska. These are fragments of consocies, 

 whose zonal relations are evident only where climatic conditions permit the 

 development of forest. Similar detached thickets of Cercocarpus occur in 

 the Wildcat Mountains of western Nebraska. Their proper relation can be 

 understood only by a study of Cercocarpus as a consocies of the foot-hills of 

 Colorado and Wyoming, where it is associated with Quercus, Rhus, and other 

 shrubs. To Quercus and Rhu^ trilobdta it bears a distinctly zonal relation, 

 since it is the most xerophytic of the three, and consequently occupies knolls 

 and ridges. The foot-hills, however, are so much dissected and bear so many 

 outcrops of rock that the fundamental zonation is greatly interrupted, and in 

 some cases thorough examination alone will disclose the fact that the niunerous 

 alternes are actually fragments of zones. The rolling character of the prairies 

 has a similar effect. Ravines, gullies, and ridges of varying extent and rank 

 are so numerous that zonation is often completely obscured and can be 

 revealed only by tracing the distribution of characteristic species. This 

 effect is enhanced by the many kinds of exposure and the ever-changing angle 

 of slope and their effect upon both migration and ecesis (plate 34 a). 



Developmental relation of layers. — Fundamentally, layers are zones related 

 to the decrease in Ught intensity from the primary layer toward the soil, 

 though the increasing shade is really the reaction of the constituent species. 

 Layering differs from zonation in being vertical instead of lateral and in giving 

 a correspondingly complex structiu'e to the conununity. Thus, while the 

 developmental relation of layers is certain, it is not obvious. It is most 

 evident in the layers of submerged, floating, and amphibious plants in water, 

 since these are of course so many developmental stages and are associated 

 only in mictia. The most typical development of layers is in forest, and this 

 alone need be considered, since the less complete layering of grassland, herb- 

 land, and scrub is fundamentally similar. In the forest with a complete set 

 of layers the latter indicate in a general way the sequence of life-form stages 

 from the ground-layer of mosses and lichens through herb, grass, and shrub 

 layers to the primary layer of trees. The species correspondence of layers and 

 stages is usually slight or none, owing to the great difference in light intensity 

 after the forest is established. However, a few species adapt themselves so 

 readily that they persist for some time during the forest climax, and play a 

 recognizable part in the constitution of layers. Such a result is indicated by 

 the reciprocal fact that some species of the forest undergrowth are able to 

 persist after the trees have been removed. In the spruce forests of the Rocky 

 Mountains, Opulaster often persists to become the dominant species of the 

 shrubby layer. In the final maturing of the spruce forest the number of 

 layers is directly dependent upon the increasing density of the crown, and 

 hence serves as a ready index of the degree of matiirity, i. e., of development. 



