92 CARNEGIE INSTITUTION OF WASHINGTON. 



Torrens. The abundance of halophytes makes possible the very con- 

 siderable pastoral industry of the far north. 



A list of particular adjustments of the species to the environment 

 would include, among others, the following: The leaf-surface is usually 

 greatly reduced and aphyllous species are common. In the acacias 

 and in certain cassias the true leaves may be replaced by phyllodia. 

 The leaves may be heavily cuticularized and often they are coated 

 with resinous substances, especially when young. In many instances 

 leaves and young stems are covered with trichomes, although such 

 covering is not a striking character of the species as a whole. Water- 

 storage capacity is present in the roots of certain species, as Gravillea 

 stenohotrya and Eucalyptus sp. In subarid regions species of Eucalyp- 

 tus with shrub habit, the "mallee," have shortened stems which are 

 important water-storage organs. Succulence among non-halophytic 

 perennials is wanting or rare. The superficial roots of several species 

 are especially well developed and point to a response on the part 

 of the species to relatively light rains. In the sandhills and along 

 water-courses the superficial roots may attain great length. Where 

 water-storage capacity is present in roots it is usually situated in, if 

 not confined to, such as are superficially placed. Many small trees 

 have canopy-formed shoots. This is a striking character of species of 

 "mallee," but occurs in species of other genera as well. Vegetative 

 habit of reproduction through stolon-like subterranean organs is fre- 

 quently met in shrubs and trees. 



Reactions of Roots of Species with Dissimilar Habitats to Different Amounts of 

 Carbon Dioxid in the Soil, by W. A. Cannon. 



It has previously been reported^ that the roots of seedling Prosopis 

 velutina and of cuttings of Opuntia versicolor exhibit an unlike reaction, 

 as shovrn by a modification of the rate of growth of the roots, to an 

 excess of carbon dioxid in the atmosphere of the soil. In the case of 

 Prosopis, whose roots may penetrate deeply, a relatively great tolerance 

 to carbon dioxid is shown. But as regards Opuntia, with superficially 

 placed roots, the tolerance is not so marked. The difference between 

 the two species in the root-carbon-dioxid relation, however, is not 

 supposed to be of definitive importance in the economy of either species. 

 The reactions of roots in relation to carbon dioxid of the atmosphere 

 of the soil of two additional Arizona species, which occur under natural 

 conditions somewhat similar to those of the two species above referred 

 to, and of a third species whose habit of growth and whose habitat are 

 widely different, are described below. 



The plants in question are Covillea tridentata and Krameria canescens, 

 from southern Arizona, and Mesembryanthemum sp. from the neighbor- 

 hood of the Coastal Laboratory, Carmel, California. So far as the habit 



1 Carnegie Inst. Wash. Year Book for 19 IG, p. 74. 



