with limber pine and balsam fir as secondary species. Vaccinium is usually abundant on the forest floor. A considerable amount of grassland occurs as mountain meadows in the upper part of this zone. "Wind timber" runs up in tongues to various altitudes and there are small patches of such scrub isolated from the main part below. Numerous small lakes and bogs occur along stream courses. (5) Alpine zone, altitude 11,500 to 14,000. This is a grassland and rock-desert zone. Dwarf willows occur, often in large patches, but there are no other woody plants.

LIST OF SPECIES OF WOODY PLANTS

PINACEAE, PINE FAMILY

Pinus flexilis James. LIMBER PINE, WHITE PINE

Usually a rather small tree but reaching considerable size among Engelmann spruces at about 10,000 ft. At lower altitudes it occurs chiefly in wind-swept stations. Easily distinguished from other pines of this county by its large, smooth cones and its short leaves in bundles of five. In exposed places it is much contorted, the trunk sometimes prostrate.

Altitude: 7,500 to 11,000 ft. Collections: Eldora; Sugarloaf; Ward. Isolated specimens of this pine probably occur as low as 6,500 ft. where much exposed to wind. Pinus murrayana "Oreg. Com." LODGEPOLE PINE

A slender medium-sized tree forming dense, pure forests at altitudes of about 8,000 or 9,000 ft. It also occurs mixed with the rock pine and limber pine in some places. Leaves are in bundles of two, the cones small and numerous. Used for fuel, timber, posts and railroad ties.

Altitude: 6,500 to 10,500 ft. Collections: Allenspark; Eldora; Pine Glade School. Pinus scopulorum (Englem.) Lemmon. Rock PINE

A handsome spreading tree, the largest of our native conifers. This is the only pine on the mesas and lower foothills in the county. Formerly much used for lumber but the supply in Boulder county now nearly exhausted. The lumber is known locally as "native" and "black jack." Easily distinguished by its long leaves, 1 dm. or more in length. The leaves are in bundles of two or three.

Altitude: 5,000 to 9,000 ft. Collections: Eldora; Sugarloaf; Bluebell Canyon; head of Gregory Canyon, altitude 7,400 ft.

Picea parryana (Andree) Sarg. BLUE SPRUCE, Silver SprucCE

A handsome, ornamental tree sometimes with silvery bloom especially on the leaves at tips of branches. It should be noted, however, that the Engelmann spruce and the Douglas spruce frequently are "blue" also. Moist situations along streams throughout the county within its altitudinal limit. The blue spruce never forms forests but occurs in fringes along streams or sometimes in small groves. Much planted for ornament.

Altitude: 6,500 to 10,500 ft. Collections: Fourth of July Mine; South Boulder Canyon, altitude, 6,800 ft.

country, with an altitude of from 4,900 to 5,500 ft. To the west is a region of foothills, dissected plateaus and alpine peaks. The peaks vary in height from 12,500 ft. (Bald Mt.) to over 14,000 ft. (Long's Peak). The foot of the main range, or continental divide, is 10,000 to 10,500 feet in altitude and the lowest passes from 1,000 to 2,000 ft. higher. Boulder County is in north central Colorado, the city of Boulder being about 25 miles northwest of Denver.

Altitudinal distribution.-The limits of altitude printed in the systematic list are based not only on collections, but also on field notes taken at various times when no collections were made. It is believe that they are substantially correct. However, occasional plants of nearly every species may be found somewhat above or below the limits given. This is naturally the case since a number of edaphic factors may unite in places to overbalance the climatic factor of temperature which is the chief one introduced by altitude. It is worthy of note that most of the trees of the plains region ascend only to 6,000 or 6,500 ft.; a few of the trees and a number of shrubs reach 8,000 or 9,000 ft. Above 9,000 ft. there is considerable change in the flora and at an altitude of 10,000 ft. practically all of the woody plants are true mountain species. If, then, one wishes to go from the plains at Boulder (or Denver) to where there is an entirely different flora it will be necessary to reach an altitude of about 10,000 ft. The chart on page 52 shows the altitudinal distribution for a number of our most characteristic and easily identified species of woody plants.

Zones of plant life. The following zones of plant life in Boulder County may be distinguished.1 (1) Plains zone, altitude up to 5,800 ft.—a grassland formation; trees and shrubs only along streams and on rock ridges. (2) Foothill zone, altitude 5,800 to 8,000 ft.—generally a rather open forest of rock pine on hillsides with a mixture of Douglas spruce on north slopes and some deciduous trees in the canyons. (3) Montane zone, altitude 8,000 to 10,000 ft.-closer forest of lodgepole pine with admixtures of rock pine and limber pine at various places; Engelmann spruce in the higher parts. (4) Sub-alpine zone, altitude 10,000 to 11,500 ft.-characterized by forests of Engelmann spruce

See a note on this subject by the present writer in Science, Vol. XXVI, p. 642, 1907.

with limber pine and balsam fir as secondary species. Vaccinium is usually abundant on the forest floor. A considerable amount of grassland occurs as mountain meadows in the upper part of this zone. "Wind timber" runs up in tongues to various altitudes and there are small patches of such scrub isolated from the main part below. Numerous small lakes and bogs occur along stream courses. (5) Alpine zone, altitude 11,500 to 14,000. This is a grassland and rock-desert zone. Dwarf willows occur, often in large patches, but there are no other woody plants.

LIST OF SPECIES OF WOODY PLANTS

PINACEAE, PINE FAMILY

Pinus flexilis James. LIMBER PINE, WHITE PINE

Usually a rather small tree but reaching considerable size among Engelmann spruces at about 10,000 ft. At lower altitudes it occurs chiefly in wind-swept stations. Easily distinguished from other pines of this county by its large, smooth cones and its short leaves in bundles of five. In exposed places it is much contorted, the trunk sometimes prostrate.

Altitude: 7,500 to 11,000 ft. Collections: Eldora; Sugarloaf; Ward. Isolated specimens of this pine probably occur as low as 6,500 ft. where much exposed to wind. Pinus murrayana "Oreg. Com." LODGEPOLE PINE

A slender medium-sized tree forming dense, pure forests at altitudes of about 8,000 or 9,000 ft. It also occurs mixed with the rock pine and limber pine in some places. Leaves are in bundles of two, the cones small and numerous. Used for fuel, timber, posts and railroad ties.

Altitude: 6,500 to 10,500 ft. Collections: Allenspark; Eldora; Pine Glade School. Pinus scopulorum (Englem.) Lemmon. ROCK PINE

A handsome spreading tree, the largest of our native conifers. This is the only pine on the mesas and lower foothills in the county. Formerly much used for lumber but the supply in Boulder county now nearly exhausted. The lumber is known locally as "native" and "black jack." Easily distinguished by its long leaves, 1 dm. or more in length. The leaves are in bundles of two or three.

Altitude: 5,000 to 9,000 ft. Collections: Eldora; Sugarloaf; Bluebell Canyon; head of Gregory Canyon, altitude 7,400 ft.

Picea parryana (Andree) Sarg. BLUE SPRUCE, SILVER SPRUCE

A handsome, ornamental tree sometimes with silvery bloom especially on the leaves at tips of branches. It should be noted, however, that the Engelmann spruce and the Douglas spruce frequently are "blue" also. Moist situations along streams throughout the county within its altitudinal limit. The blue spruce never forms forests but occurs in fringes along streams or sometimes in small groves. Much planted for ornament.

Altitude: 6,500 to 10,500 ft. Collections: Fourth of July Mine; South Boulder Canyon, altitude, 6,800 ft.

Picea engelmanni (Parry) Engelm.

ENGELMANN SPRUCE

A handsome conical tree frequently forming pure forests at about 10,000 to 11,000 ft. Common everywhere in the higher altitudes throughout the county. It forms most of the "wind timber" at timber line. Used for timber and railroad ties. The lumber is known locally as "white spruce."

Altitude: 8,500 to 11,500 ft. Collections: Eldora, Silver Lake. Pseudotsuga mucronata (Raf.) Sudw. DOUGLAS SPRUCE

A handsome broad-based conical tree of moist gulches and north slopes. It has a less compact and more feathery appearance than the true spruces. The wood is valuable

CHART ILLUSTRATING ALTITUDINAL DISTRIBUTION The figures at the left indicate altitude above sea level expressed in feet. The lowest point in Boulder County is about 4,900 ft. Probably most of the species which are indicated as extending down to 5,000 ft. really reach to the lowest point in the county. The species selected for the chart are those most easily recognized without attention to minute characters.

for lumber and when used locally is known as "red spruce." Young trees are cut for decoration at Christmas time.

Altitude 6,000 to 10,000 ft. Collections: South Boulder Canyon; Gregory Canyon; Sugarloaf Mountain; Eldora.

Usually a small tree, but sometimes 3 to 4 dm. in diameter. In the lower parts of

its range this tree grows with limber pine and at higher altitudes with Engelmann spruce. Scattered trees are found to the upper limit of wind-blown timber on all the peaks. Seldom used for lumber; the wood is weak and brittle.

Altitude: 8,000 to 11,500 ft. Collections: Eldora; Ward; foot of Mt. Audubon.

JUNIPERACEAE, JUNIPER FAMILY

Juniperus sibirica Burgsd. JUNIPER

A prostrate spreading shrub with sharp-pointed leaves. Dry situations on mesas and foothills to the base of the main range.

Altitude: 6,000 to 10,000 ft. Collections: South Boulder Canyon, 6,800 ft.; above Magnolia; Sugarloaf; Eldora; foot of Long's Peak; Gresham; "mountains between Sunshine and Ward" (Rydberg).

Sabina scopulorum (Sarg.) Rydb. CEDAR, Red Cedar

A small conical tree, or in wind-swept places much distorted. Usually in exposed situations on hills and ridges; formerly used for fence posts, but the supply now exhausted. Altitude: 5,000 to 8,500 ft. Collections: Bluebell Canyon; South Boulder Canyon 6,800 ft.; above Magnolia; Sugarloaf.

SMILACEAE, SMILAX FAMILY

Nemexia lasioneuron (Hook.) Rydb. SMILAX, CARRION FLOWER

A low, trailing plant of shady situations growing under trees and bushes.

Altitude: 5,000 to 6,500 ft. Collections: mouth of Boulder Canyon; mouth of Gregory Canyon.

SALICACEAE, WILLOW FAMILY

Populus acuminata Rydb. LANCELEAF COTTONWOOD

A medium-sized tree of somewhat symmetrical form rather rare in Boulder County except where planted. Along streams.

Altitude: 4,900 to 6,000 ft. Collections: gulch one mile south of Chautauqua; Valmont; cultivated at Boulder.

Populus angustifolia James. NARROWLEAF COTTONWOOD

A tree of open, unsymmetrical form growing along streams. Very common. Altitude: 5,000 to 9,000 ft. Collections: 17th Street bridge at Boulder; Boulder; South Boulder Canyon; Valmont; mouth of Gregory Canyon; "Boulder" (Rydberg). Populus balsamifera Linn. BALM OF GILEAD, BALSAM POPLAR

A medium sized tree of gulches and along streams.

Altitude: 8,000 to 10,000 ft. Collections: Fouth of July Mine; Eldora; Allenspark.

Populus sargentii Dode. COMMON WEstern CottoNWOOD

This is Populus occidentalis of Rydberg's Flora and P. angulata of Coulter's Manual. Common along streams in the plains region and occasional in the mountains. Much planted for shade.

Altitude: 4,900 to 8,500 ft. Collections: Valmont; Boulder Creek; Boulder; mouth of Gregory Canyon; "Eldora to Baltimore" (Rydberg).

Populus tremuloides Michx. QUAKING ASPEN

A small tree of moist slopes and gulches forming characteristic groves.