92 THE POPULAR SCIENCE MONTHLY 



probably most of the rootlets formed in any season perish before the 

 close of the succeeding dry season. 



As regards the second class of roots referred to, there is quite a 

 different story to tell. These roots are apparently quite as deciduous 

 as the leaves of many plants. It should be noted, however, tbat noth- 

 ing has been done in an experimental way to test the longevity of these 

 roots, and it is reasonable to suppose that under some favoring condi- 

 tions they might endure, possibly becoming converted into large laterals, 

 even if under conditions, which are the usual ones, their life is limited. 



It may be well to describe the roots referred to. If we examine a 

 root of such a desert shrub as Franseria, we shall find, along such of the 

 roots as extend in a more or less horizontal direction from the stem of 

 the plant, groups of filamentous rootlets. These occur at about 1 cm. 

 intervals, in varying numbers usually about one half dozen together. 

 They are from two to four cm. long and probably not more than one 

 half millimeter in thickness. The rootlets appear promptly with the 

 coming of the summer rains, and they cease their activity when the soil 

 attains to an unbearably dry condition, as perhaps in adobe soil, 10 per 

 cent, moisture, more or less. 



The deciduous rootlets greatly increase the absorption surface with- 

 out, at the same time, necessitating invasion of new root areas, or of 

 causing a long transfer of water from the place of absorption to the 

 stem. So far as is known, the deciduous rootlets are formed only when 

 there is an abundance of water, and when the temperature is high. 

 These rootlets have been seen on most of the desert shrubs, on all in the 

 vicinity of the Desert Laboratory, and have been observed on a few of 

 the shrubs in southern Algeria. "Whether a similar kind of rootlets 

 occurs on perennials in the more humid regions is not known to me. 



The deciduous rootlets are thus of great importance to such desert 

 plants as bear them. They appear adventitiously always, and appar- 

 ently in the same place on the root year after year. In certain species 

 it has been observed that the adventitious roots are formed precociously, 

 but in other forms this is not the case. And again, where such rootlets 

 are not to be found, it appears that they can not be induced. 



The extension of the roots of the desert shrubs is various, perhaps 

 in no case exceeding three or four meters. The position in the ground 

 is also not uniform. In most instances the position occupied by the 

 roots is characteristic for the species, but it is likely that the extension 

 of the root-systems varies mainly with the age of the individual. 



There are three main types of root-systems to be found in the shrubs 

 of the desert plants of the southwest. (1) Root-systems which extend 

 horizontally from the main plant axis and lie, for their whole course, 

 near the surface of the ground. (2) Root-systems which are charac- 

 terized by a strongly developed tap root going directly down to a depth 



