GENERAL DISCUSSION. 177 



Three main types of plants, however, are distmguishable as being characteristic of 

 arid areas and of the included localities in which the soils are highly charged with salts: 

 the spinose xerophytes, which are generally woody shrubs, with reduced branches and small 

 and indurated leaves, although some herbaceous forms are to be included; the succulents, 

 in which roots, stems, and leaves have been variously modified by the exaggeration of 

 parenchymatous tracts, so that they may hold a large amount of surplus water; and the 

 halophytes or species inhabiting saline areas, many of which also carry a large balance 

 of water. 



The spinose xerophytes are perhaps the most widely prevalent of all of the plants 

 characteristic of arid regions. Alhagi, Parkinsonia, Holocantha, Condalia, Zizyphus, etc., 

 are examples of the group and these plants show many morphological and physiological 

 qualities having the force of adaptations or accommodations to the environment, though a 

 too direct causal connection is not to be taken for granted. Plants of this type are found in 

 all regions in which arid climates of any type prevail — in the Cahuilla, in the Sahara where 

 the rainfall is low and uncertain, as well as in regions like that around the Desert Laboratory, 

 in which well-defined periods of precipitation come with a certain regularity. Examples 

 of this type are also found in localized dry habitats in regions with a comparatively moist 

 climate. It is notable that spinose types make up the few species which are to be found 

 in the most arid regions in which the supply of moisture all comes from underneath and 

 the evaporating capacity of the air is highest. The roots are in constant absorbing contact 

 with the soil at all times, and although the actual amount, or rate of acquisition of water 

 may be low, it is true that once absorbent contact with the soil is broken it is not easily 

 restored; hence these forms are not easily transplantable. The sap of the stems and leaves 

 of some of the spinose xerophytes has been found to show a concentration of over 100 atmos- 

 pheres, which is far higher than that of ordinary mesophytic plants, and the suggestion lies 

 near that this condition in the shoot might be a means of damage to the absorbing organs 

 of uprooted plants and that the transpiration stream once broken is not to be restored readily 

 under such extreme conditions, which do not extend to include the roots under ordinary 

 circumstances. 



The succulents are most abundant in regions in which the precipitation comes within 

 well-defined seasons with fair regularity, and hence are not to be expected in the Cahuilla 

 or in such climates as those of the Sahara. They are, however, a very important element 

 of the flora of the region about the Desert Laboratory, parts of South America, and eastern 

 and southern Africa. Cacti, several genera of the Asclepiadacese, Euphorbiacese, the Yuccas, 

 Amaryllidacese, etc., furnish examples in which tracts of parenchyma in various members 

 become greatly exaggerated, and in which a great balance of water accumulates. The 

 root-systems of such plants, so far as they are known, appear to have an absorbent contact 

 with the soil only during the rainy season, when the moisture content is high, and the con- 

 centration of the soil solution is lowest. The inequality between the concentration of the 

 sap of the shoot and the roots, which was noted in the spinose plants, is not present, as 

 the juice of the cacti does not usually show an osmotic pressiu^e of more than 10 or 12 

 atmospheres. In contrast with the continuous absorbent action of the roots of the spiny 

 types, many of the succulents actually undergo a decortication of the root-system during 

 the dry seasons, so that the taking in of water in any quantity is impossible until new 

 rootlets have been formed. This usually takes place very quickly with the coming of the 

 rains. 



It is to be taken for granted that the succulents are subject to the same high evapora- 

 tive action of the air as the spinose forms of the regions which they inhabit. It is therefore 

 to be expected that their external structiu-e and morphology would include some of the 

 features characteristic of the spinose forms. As a matter of fact many of these are exhibited 

 in more pronounced or highly developed stages. Induration, thickening, or heavy coatings 

 12 J 



