EFFECTS OF CERTAIN ECOLOGICAL FACTORS 



Certain ecological factors can exist only under certain co«n- 

 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 pedc^ncles 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 cf 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 (car)3cnates 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 mors 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. Ihees all have flat and ampje 

 leaf surface. Similar plants of th'^ 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, tetrapteriis, all 

 have greatly reduced leaf surfaces, the leaflets mostly reduced to 

 phyllodia and the stems, petioles i.nd. 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-like 

 leaves of the Podo-sclerocarpi. Such species are A. Episcopus, Wood- 

 ruff!, 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 rale. The normal rainfall 

 of the Great Basin is twice that of the Navajo; the normal tem- 

 perature 2 to 4 degrees less. Strongly alkaline places in the Tropical 

 life zone rarely have perennial species of Astragalus. Gen -rally 

 such species are annuals whose roots do not penetrate the 5 irface 

 sands, far, and so the effects of alkalies are not so evident. 



