tangential and also by radial lines, which latter appear to be continuations of the middle lamellas between the cells, (fig. 3.) The tangential lines are clearly due to stratification of the substance produced by successive changes in its composition affecting its refractive index, for they are progressively further apart and fainter, and the presence of the radial lines clearly indicating the lateral limits of each cell, shows that the secretion or modification of each cell takes place independently of those of contiguous cells.
The secretion does not take up any of the ordinary protoplasmic stains, such as eosin, osmic acid, or hsematoxylon ; nor could we obtain any reaction with methyline-blue, Schultz solution or iodine.
We conclude that the secretion is due to a modification of the cell-wall, and though controlled and brought about by each cell independently, is not a direct product of its protoplasm. But that it is not a gum or mucilage in the ordinary sense is shown by the fact that it does not swell and dissolve in water at the ordinary temperature, nor take up any cellular stains nor show any cellulose reaction .
The Root-hairs.
As stated above, the root-hairs are found only on parts of the root near the surface of the sand. A few inches down they are cut off by the exodermis, which is formed as usual in monocotyledons, and shrivel up. But near the surface there is no exodermis, and the root-hairs formed while that part was still young persist apparently indefinitely.
The Regions of the root.
There are three regions in the root : —
I. From the tip to the region of the exodermis the surface is covered by the secretion. This may be two to six inches long and the surface is white, smooth and glistening.
II. In the region of the exodermis the surface tissue is dead and brown. It is in this region that the short rootlets are found.
III. From the exodermal region to the surface, for a length of two or three inches as a rule, the piliferous layer remains fresh, the root-hairs persist and adhere firmly to grains of sand.
The existence of a secretion by the piliferous layer has been noted, we believe, in only a few grasses, which grow in the Algerian Sahara. The chief of these is Aristida pungens. R. Price (2) who has examined the secretion, pointed out that in Aristida it binds together the sand particles with a tubular sheath into which root-hairs run and are thus enabled, probably, to take up moisture from a larger volume of sand.
