ea re eh ee he ee en ee ee 
1906] SHANTZ—VEGETATION OF THE MESA 35 
Muhlenbergia gracillima Torr., Agropyrum occidentale Scrib., (Koeleria cris- 
tata [L.] Pers.). 
PRINCIPAL SPECIES: Artemisia frigida Willd., (Thelesperma intermedium 
Rydb.), (Schedonnardus paniculatus [Nutt.] Trel.), Gutierrezia Sarothrae 
(Pursh) Britt. & Rusby, (Grindelia squarrosa [Pursh] Dunal), Chrysopsis 
villosa (Pursh) Nutt., (Eriogonum annuum Nutt.), (Artemisia canadensis 
Michx.), Lupinus so Pursh), (Carduus plattensis Rydb.), Aristida 
longiseta Steud., Psoralea tenuiflora Pursh., Aecidium psoralea Pk., Boebera 
papposa (Vent.) Rydb., Plantago Purshii R. & S., (Selaginella inks Rydb.), 
(Eriogonum effusum Nutt.), (Sporobolus cryptandrus [Torr.] Gray), (Eriogonum 
annuum Nutt.) 
SECONDARY SPECIES: Oreocarya thyrsiflora Greene, Petalostemon oligo- 
phyllus (Torr.) Rydb., P. purpureus (Vent.) Rydb., Euphorbia glyptosperma 
Engelm., Thelesperma gracile (Torr.) Gray, Gaura coccinea Pursh, Tetra- 
neuris glabriuscula Rydb., Malvastrum coccineum (Pursh) Gray, Atheropogon 
curtipendulus (Michx.) Fourn., Sideranthus spinulosus (Nutt.) Sweet, Lacini- 
aria punctata (Hook.) Kuntze, Eriogonum Jamesii Benth., Munroa squarrosa 
(Nutt.) Torr., Euphorbia serpyllifolia Pers., Chenopodium leptophyllum (Moq.) 
Nutt., Mentzelia nuda (Pursh) Torr. & Gray, Argemone intermedia Sweet, Phy- 
salis comata Rydb., Atriplex argentea Nutt., Potentilla pennsylvanica L., Asclepias 
pumila (Gray)Vail, Bouteloua prostrata Lag., Helianthus pumilus Nutt., Sitanion 
elymoides Raf., Muhlenbergia gracilis Trin., Artemisia gnaphalodes Nutt., Stipa 
comata Trin. & Rupr., Stipa Vaseyi Scrib., Allionia linearis Pursh, Artemisia 
canadensis Michx., Leucolene ericoides (Torr.) Greene, Picradeniopsis oppositi- 
folia (Nutt.) Rydb., Hedeoma nana (Torr.) Greene, Stipa neo-mexicana (Thurb.) 
Scrib., Chenopodium oblongifolium Wats., Anogra coronopifolia (T. & G.) 
Britton, Leptilon canadense (L.) Britton, Helianthus annuus L., Andropogon 
furcatus Muhl., Solanum rostratum Dunal, Ptiloria ramosa Rydb., Cheno- 
podium album L., Helianthus petiolaris Nutt., Quincula lobata (Torr.) Raf., 
Grindelia squarrosa nuda (Wood) Gray, Carduus undulatus Nutt., Carduus 
plattensis Rydb., Gaura parviflora Dougl., Potentilla coloradensis Rydb., Eri- 
ogonum alatum Torr., Euphorbia stictospora Engelm., Mentzelia decapetala 
(Pursh) Urban & Gilg, Andropogon scoparius Michx. 
An increase in temperature, a decrease in rainfall and relative 
humidity, together with the resulting decrease in water content 
of the soil mark the appearance of the aestival period. The per- 
centage of water in the soil is for this period 10-13 per cent. for 
clay and 2.5-4.5 per cent. for gravel. 
This aspect is characterized by the flowering of most of the annual 
species and by the predominance of the grasses and composites. 
The facies of the formation are the dominant species of this aspect 
