Alaska District. 359 
of the 56° N.L., forming in the valley of the Yukon forests of no little im- 
portance. Betula papyrifera, Populus balsamifera, Pinus Murrayana and 
Abies subalpina occur here (Betula-Populus- -Facies),. The larch Zarir 
americana is known from only one locality in Alaska, Fort Gibbon, mouth of 
the Tanana River. 
In these woods are found Botrychium virginianum, Nephrodium spinulosum and dryopteris 
(on stony soil), Polypodium vulgare, Pellaea (Cryptogramme) Stelleri, Cryptogramme acrostichoides 
(between stones), Pteridium aquilinum ‚ Cystopteris fragilis, Streptopus roseus, Linnaea borealis, 
Cornus canadensis, Arabis Holboellii, Lady ochroleucus, Arctostaphylus uva-ursi and Vaccinium 
vitis-idaea. From the accounts of explorers the interior appears to be an open platean devoid 
On the headquarters of the Yukon about Pelly River the mountains, a 
short distance from the river, are covered with a heavier growth of spruce, 
while the Populus balsamifera, Alnus incana and viridis with Salıx cordata, 
Viburnum, Cornus, Ribes, Rosa nutkana, Spiraea salicifolia and Potentilla 
Jruticosa line the river banks and the birches are scattered. The exposed 
hillsides are covered with boreal sage brush (Artemisia frigida) with here 
and there a prostrate juniper, or a small clump of spruces, terminating here the 
Artemisia frigida-Formation. The Yukon widens after it receives the waters 
of the Big and Little Salmon rivers. The rolling hills are sometimes a mile 
or several miles from the river bank with low willow swamps between. 
Alluvial Island Formation. Islands in the river, however, are covered 
with a luxuriant vegetation. Alders generally form the outer strip; next come 
the willows; next the poplars, rising somewhat higher; and finally the dark- 
green spruces which occupy the central area. As undergrowth occur plants 
of the genera Cornus, Viburnum, Rosa, Ledum, Vaccinium, Ribes and She- 
pherdia. Pinus Murrayana occurs though not forming a continuous forest 
and it disappears entirely beyond Fort Selkirk. 
After leaving the junction of the Pelly River the poplars of the forest 
increase over the spruces not only in number but in size, while the birches 
hold their own and Pinus Murrayana is entirely absent (Populus-Facies). 
Finally Picea alba becomes dwarfed and entirely subordinate to the poplars 
which form a dense forest crown for miles. The spruces occupy the gulches 
and occur as small clumps elsewhere. The undergrowth remains the same, 
and deep moss covers the ground and rocks. Juniperus nana, F. sıbirica is 
found occasionally on hillsides and in more open hilly places the landscape 
is brightened by Zpzlobium (Chamaenerion) angustifolium. The sandy islands ne 
of the lower Yukon River are covered with impenetrable thickets of willow, a 
while the hills are covered with poplars, alders and birches in dense thic 
with scattered spruces. 
i.e. of the Matanniska River and affluents are: 
Anemone narcissiflora L. Aconitum delphinifolium DC. a 
» parviflora L. Arabis Iyrata L. ER 
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A few of the remarkable plants of the ze basin 2% 
