182 BOTANICAL GAZETTE [SEPTEMBER 
of the thicket formation. Among the first species to appear in the 
shade of these advancing shrubs are Calochortus Gunnisonit, Mer- 
tensia linearis, and Stipa neo-mexicana. Later, when the habitat 
has been rendered less xerophytic and when the other facies have 
entered, Coleosanthus umbellatus, Oryzopsis micrantha, Elymus con- 
densatus, Selaginella rupestris Fendleri, Filix fragilis, and a number 
of other species appear. . 
- Under perfectly natural conditions and without the intervention 
of herbivora, the thicket would undoubtedly replace the greater part 
of the grass near the mountains, but, as is seen later, the thicket is 
slowly giving way and the grass formation is advancing. 
PLAINS RUDERAL FORMATION. 
Factes: Stipa Vaseyi Scribn., Puccinia Stipae Arth., Boebera pappos@ 
(Vent.) Rydb., Helianthus petiolaris Nutt., Puccinia Helianthi Schw., Thelesperma 
intermedium Rydb., Verbena bracteosa Michx., Amaranthus blitoides S. Wats. 
PRINCIPAL SPECIES: Salvia lanceolata Willd., Lappula occidentalis (Wats.) 
Greene, Polygonum aviculare L., Erysiphe Polygoni DC., Munroa squarrosa 
(Nutt.) Torr., Salsola Tragus L., Euphorbia glyptosperma Engelm.., Malvastrum 
coccineum (Pursh) Gray, Puccinia Malvastri Pk., Vicia americana, Aecidium 
porosum Pk., Solanum rostratum Dunal, Helianthus annuus L., Cleome serrulata 
Pursh, Schedonnardus paniculatus (Nutt.) Trelease, Atriplex argentea Nutt., 
Senecio spartioides Torr. & Gray, Verbesina encelioides (Cav.) Gray; Helianthus 
petiolaris Nutt., Puccinia Helianthi Schw., Picradeniopsis oppositifolia (Nutt.) 
Rydb. : 
The secondary species of this formation are very numerous. 
The physical factors of this formation are practically the same 
as those given for the Bouteloua formation. The conditions of 
water content are such that the most common ruderals, easter and 
European species, cannot thrive to the best advantage. Many 
native plants behave as ruderals and this name is applied to the 
formation. This formation represents many different stages in be 
succession which will result ultimately in the grass formation, and 1s 
always invaded by the plants of the grass formation. Only @ 7° 
of the ruderal species succeed in the grass formation. The most 
important of these is Boebera papposa, occurring everywhere through 
out the formation and sometimes very abundant. It is much better 
developed in a ruderal position and is regarded as a part of the 
ruderal formation. Stipa Vaseyi, another native plant, succeeds 
best in ruderal positions, but is often found as part of the formation- 
