Roots of some North African Desert-Grasses. 335 
drawn from a portion of material thus treated and shows the long 
root-hairs. 
As already mentioned, sections of a branching knot which 
exhibited a structure certainly quite as mature as that from which 
Text-fig. 2 was drawn, and which were of greater diameter, failed to 
show any signs of hairs. Otherwise the hairs appear to be quite 
ubiquitous and persistent. 
The processes which take place as one of the roots of A. pungens 
grows through the soil is probably much as follows. The mucilaginous 
root-cap allows the root to pass easily through the sandy soil and 
prevents its tip from drying up. The layer of mucilage behind the 
tip comes into contact with the soil and sand grains are embedded 
in it. The mucilage gradually spreads round the grains and comes 
into contact with others, while the root expands. When the root 
has reached its full diameter the complete sheath formed of sand 
grains cemented together closely invests it. The cells of the 
piliferous layer now grow out into the root-hairs which embed 
themselves in this sandy sheath. 
The root-hairs themselves obviously do not secrete the mucilage, 1 
but simply serve as absorptive organs in the usual way. 
Biologically, this root system seems specially well adapted for 
obtaining the least traces of water from the arid and very permeable 
surface soil of the sandy desert, and also it is able quickly to absorb 
the water resulting from slight precipitation. The roots of a single 
plant radiate through a large area of the surface sand, and by their 
special adaptation are enabled to obtain water from all points of 
this area with which they come in contact. Thus they differ 
markedly from the roots of ordinary land plants, for these, even if 
they occupy a large volume of soil, can only absorb water through 
their tips and the tips of their branches. The intimate relation of 
sand, mucilage and root-hairs must also naturally promote water 
absorption. The mucilage itself has a high absorptive power for 
water, and transfer of water can easily take place in the mucilage 
system, so that the whole cylinder of sand in the vicinity of the 
sheath is very completely drained. 
The sheath of mucilage and sand also undoubtedly serves 
another important function—that of protection. 2 In times of 
intense drought and great heat it is almost certain that the root- 
hairs would be shrivelled if they extended freely into the sand, and 
as they are permanent organs it is more than ever important to the 
1 Volkens, l.c., p. 26. 
* Volkens, l.c., p. 26. 
