"209. | 
XLIV. —NOTE ON THE ROOT-HAIRS OF AZOLLA PINNATA, 
BY Proressor M‘NAB, M.D., F.L.S. 
{ Read November 17th, 1879.} 
During the past summer the very interesting little Rhizo- 
carpous plant, Azolla pinnata* has been growing in abund- 
ance at the Botanical Gardens, Glasnevin. An opportunity was 
thus afforded of studying the plant in a living state, and I now 
wish to record one observation made on the roots in the small 
Physiological Laboratory at Glasnevin. Strasburger in his 
splendid memoir, “ Ueber Azolla,” figures in Fig. 67, a root of 
A. filiculoides Lam. showing the outer and inner layer of the root 
sheath, but representing no root-hairs Indeed he does not describe 
any root-hairs occurring in these plants at all,and in describing the 
trichomes produced by this plant mentions only those in the back 
of the punctum vegetationis in the hollows containing Nostoc 
filaments. At page 51 of his memoir he states that in the old 
roots of Azolla pinnata, the sheath dissolves itself into its indi- 
vidual fibres, and thus gives to the root a plumose appearance. 
Surely some mistake has been committed here, because in all the 
roots of Azolla pinnata examined the sheath and hairs could be 
distinctly seen, and the connexion of the latter with the epidermal 
cells readily traced. Moreover the root-hairs are produced inside 
the sheath, and are at first held down, with their apices pointing to 
the apex of the root, until by the elongation vf the root and 
withdrawal of the sheath they are liberated. Another point of 
importance is that the root-hairs are produced in regular verticils 
or whorls, apparently from each cell of the epidermis. As the 
root elongates the cells of the epidermis divide, and the root- 
hairs become more and more widely separated from each other, 
and their original regular arrangement becomes lost 
On older roots, or those that have been injured, every trace of 
root-cap is lost, and a number of root-hairs are visible at the apex. 
The old roots are readily detached from the stem, and leave 
peculiar rounded cavities, with the central mark of the fibro- 
vascular bundle, a condition not unlike that occurring in some 
fossil plants, distantly allied to Rhizocarpee. 
* The plant is probably"4. Caroliniana, but as yet no ripe fruit has been observed. 
Scien Proc., R.D.S. Vou. u., Pr. v. x 2 
