§ ete eer 7 ., 
‘ A 
Histological Studies in the Artemisia Formation 3 
such an aggregation of plants as this, or, to formally define it, a 
plant formation is the collection of plants present in a habitat 
more or less clearly delimited, usually by certain physical condi- 
tions, in which the arrangement and the structure of the plants 
is the result of the adaptation of these plants to the environic 
forces of the habitat, and further characterized by the predom- 
inance of one or several species. To localize for the Artemisia 
formation it only remains to be briefly stated that this formation 
is characterized by a low water-content, high temperature, low 
humidity, xerophytic vegetation, and by the predominance of 
Artemisia tridentata, 
The geographical limits, the structure, and the development of 
the formation are not treated in this place, but are reserved for 
a future paper, when a more comprehensive reconnaissance of 
the whole formation shall have been made. 
THE REGION IN WHICH THE FIELD STUDIES WERE MADE 
The studies which constitute the field materials for the paper 
were made during the summer of 1907 from June 20 to August 
30, in the Artemisia formation as it is found in the state of Colo- 
rado. The southern half of the state was covered quite thor- 
oughly. The studies were ended in the north central part of the 
state. The most extensive studies were made at Fort Garland, 
Durango, Hot Sulphur Springs, and in Estes Park. A person 
familiar with the geography and physiography of the state will 
know from the above statement that the work was done mostly 
within the Rocky mountain portion of the state. The altitude of 
the formation ranges from about 1,500 m. (4,875 ft.) to 2,830 m. 
(8,500 ft.). No plants of Artemisia tridentata were found above 
an altitude of 3,000 m. (9,975 ft.). About 1,500 km. (2,500 
miles) were traveled during the summer. The outline map on 
the following page will show the route followed and the location 
of the principal studies. . 
MATERIAL AND METHODS 
As stated above only leaf studies were made, except in the case 
of Juncus in which the leaf functions are performed by the leafless 
413 
