THE BOOT-SYSTEMS OF DESERT PLANTS 9 1 



From this it is to be seen that the localities referred to, instead of being 

 typically intense deserts, are, on the other hand, the most favorable 

 situations as regards moisture. 



It is difficult, at present, to state under what conditions the roots 

 of the desert plants are formed, owing mainly to the lack of experi- 

 mental evidence. But by a system of reasoning backwards from the 

 mature root we can possibly picture to ourselves something of these 

 conditions. In the first place, if we examine the root-systems of desert 

 plants, in the field, during the season of drought, we shall find it very 

 difficult, if not impossible, to find any portions which show vegetative 

 activity, although it may be possible at the same time to demonstrate a 

 certain, even if low, rate of transpiration. On the other hand, if the 

 root-systems of the desert plants are examined during the rainy periods, 

 there will be no difficulty whatever in finding fresh growth, new rootlets 

 of whatever kind. But that this is not the whole story is evidenced by 

 the fact that in winter many of the plants native to the southwest do 

 not form new roots, or, at least, I have not been aide to find new roots. 

 In spite of this fact, such plants as the flat opuntias do, in winter, absorb 

 water and very promptly after rains. This is shown by the thickening 

 of the fleshy and flat stems. It is therefore probable that a certain 

 amount of heat as well as of moisture is required to bring about the 

 formation of fresh roots. In addition to these two factors, there is 

 probably another one, namely, aeration of the soil. Whether this is 

 mainly concerned with the formation of the roots or of the position 

 occupied by the roots in the soil is not known. It seems highly probable 

 in certain cases, particularly in fleshy plants like the cacti and some 

 liliaceous forms, that the amount of air in the soil must be of impor- 

 tance in determining the position occupied by the roots. So far as 

 observation goes, the roots formed may be classed in at least two 

 categories: (1) They constitute the extension of the roots previously 

 formed and (2) they may appear on much older roots, but are of limited 

 growth. It is supposed that in the main the greatest amount of water 

 taken into the plant comes through the roots of the first kind, so that 

 the place of water absorption as the roots grow, ever becomes farther 

 from the stem, and the problem of water transportation is ever an 

 increasingly difficult one. This last one is probably to be considered a 

 very important matter on the desert where the evaporation rate is often 

 very high, caused by the low relative humidity, by high temperature 

 and by air currents. It is conceivable that, given favorable conditions, 

 a large proportion, possibly all, of the roots of this character might 

 remain alive, but, as a matter of fact, in desert plants, as before noted, 

 it is difficult during the dry seasons to find any living roots of this class. 

 As one result of this we find that the extension of the root-systems as 

 a whole, away from the central plant axis, goes on relatively slowly, and 



