55 
EFFECTS OF CERTAIN ECOLOGICAL FACTORS 
Certain ecological factors can exist only under certain cgn- 
ditions. 
Acidity of the soil is possible only in cool regions where the 
vegetable humus is very dense, and where the 'humidity is great 
and the seasons short and sunshine below normal so far as the 
Astragaline flora is concerned. The species do not grow under 
acid ‘tropical conditions, so far as known. This therefore confines 
such species to the upper life zones in forests and meadows. These 
species have slender stems and delicate thin leaves, slender petioles 
and peduncles and delicate flowers nearly always purple, and seldom 
large. The pods are uniformly thin and mostly inflated. All the 
species are perennial with interlaced roots or root crowns and 
mostly woody crowns. 
Alkaline soil is possible only in regions of low humidity, rain- 
fall, and higher temperature and excessive sunshine. Such plants 
then always grow in the open and mostly on the floor of valleys 
having no outlets or poor drainage, or on clayey slopes where alka- 
lies are leaching out of saline formations. This is very common in 
the Great Basin and around the extensive clay deposits of the Ter- 
tiary, Cretaceous and Jurassic formations. It also is local through- 
out the floor of the Tropical life zone. The character of the alkali has 
much to do with the vegetation. About % of 1 per cent active alka- 
lies (carbonates of sodium and potassium) prohibit all growth, 
while 1.5 per cent of neutral alkalies (sulphates and chlorides of 
sodium and potassium) only are prohibitive. The Navajo Basin con- 
tains more carbonates, and the Great Basin more chlorides and sul- 
phates. 
The almost uniform effect of alkalies in Astragali is to thicken the 
stems, leaves and flowers, making them coarser, succulent and taller, 
and reduces the pubescence, but the relative quantity of moisture has 
a marked effect as in all plants elsewhere. The tendency is also 
for the flowers to be white and large, Typical alkaline species 
of the Navajo Basin are A. asclepiadoides, Pattersoni, and sabulo- 
sus, and probably ampullarius. Thees all have flat and ample 
leaf surface. Similar plants of the Mojave-Colorado river region 
are A. limatus and Preussii, with flat leaf surfaces, but purple flow- 
‘ers. The Great Basin species where the neutral alkalies are the 
More abundant are A. Pattersoni but with reduced leaf surfaces, but 
the typical species A. Toanus, Serenoi, canonis, tetrapterus, all 
have greatly reduced leaf surfaces, the leaflets mostly reduced to 
phyllodia and the stems, petioles and peduncles performing the 
function of leaves but the plants are tall or long as in the other, 
the flowers are purple, the pubescence is ashy and sparse. On 
the other hand the Homalobi of the Navajo Basin which grow in 
soil always somewhat alkaline all have the  phyllodia-ike 
leaves of the Podo-sclerocarpi. Such species are A. Episcopus, Wood- 
ruffi, pictus. They have no corresponding species in the Great Basin. 
The Ocreati of the Navajo Basin develop narrower and more folded 
and more hairy leaves as the sand in the soil increases and the 
alkali decreases, such species are A. confertiflorus, agrillosus and 
Moencoppensis. This follows the general rule, The normal rainfall 
of the Great Basin is twice that of the Navajo; the normal tem- 
perature 2 to4 degrees less. Strongly alkaline places in the Tropical 
life zone rarely have perennial species of Astragalus, Generally = 
such species are annuals whose roots do not penetrate the surface 
sands, far, and so the effects of alkalies are not so evident, = 
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