Roots of some North African Desert-Grasses . 339 
quite a general feature of the grasses of the sandy desert. Massart 
mentions this fact for Aristidapungens, A.floccosa, Panicum turgidum, 
Pennisetum dichotoinum, Danthonia Forsknlii , and also states that it 
occurs with most of the perennial grasses growing in the sand of 
the desert. From the cases dealt with in this paper it appears that 
some of the annual grasses with shorter roots also exhibit the same 
phenomenon. It is obvious at once that this is of advantage to 
the short-lived species, but in the perennials the freely exposed 
hairs and piliferous layer would certainly be dried up during the 
hot dry summer. It thus seems that the adaptation, already dealt 
with for A. pungens, A. obtusa and Lygeum Spartum, may be a 
special case of this general persistence of root-hairs, modified to 
serve the needs of a perennial plant. Some of the cases like 
Bromus tectorum show how the type may have arisen by the 
increased secretion of mucilage and the confinement of the root- 
hairs within the limits the sheath so formed. 
A. pungens probably represents the most efficient arrangement 
for obtaining water from the arid soil, in which the permanent 
underground water supply lies at a very great depth, and exhibits 
what may be regarded as a well marked “ adaptation ” to intensely 
xerophytic conditions. It is almost certainly this adaptation which 
has enabled the plant to assume a dominant place in the vegetation 
of the sandy desert. 
The type of adaptation may be contrasted with that exhibited 
by many desert plants, viz., the production of very long roots which 
descend vertically to the permanent supplies of water far below the 
surface of the soil. 
My best thanks are due to Mr. A. G. Tansley, who suggested 
the work, supplied the preserved material of A. pungens and who 
has given his advice throughout: also to Dr. C. E. Moss for the 
use of the herbarium material; and also to Mr. H. H. Thomas for 
the photograph. 
Botany School, 
November, 1911 . Cambridge. 
DESCRIPTION OF PLATE VI. 
All Figures Drawn with the Aid of a Camera Lucida. 
Figures 1—6. A. pungens. 
Fig. 1. Radial longitudinal section through root-cap and tip of A. pungens. 
Microtomcd after treatment with Hydrofluoric Acid. Note the outer cells of 
