370 Mrs Arber, On Root Development in Stratiotes aloides L. g 
II. The Structure and Development of the Adventitious 
Roots of Stratiotes aloides L. 
The rosette of aloe-like leaves, which characterises the Water 
Soldier, arises from an abbreviated stem, which is represented in | 
radial longitudinal section in P]. VIII, Fig. 1. A series of adven- 
titious roots is shown, becoming progressively younger towards the | 
stem apex. They arise at the outer limit of the central vascular || 
region of the axis. | 
Van Tieghem and Douliot* describe the young root as 4 
enclosed externally in a digestive sac arising from the stem | 
endodermis, which is followed internally by a root-cap derived | 
from the pericycle. I cannot, however, confirm this description, | 
as it appears to me quite impossible to demonstrate that the | 
digestive sac is cortical and the root-cap stelar in origin, since no 
distinct endodermis and pericycle can be seen in the stem, and | 
there is also no visible distinction between root-cap and digestive 
sac in the root. My observations agree with those of Miss D. G. 
Scott+, who also failed to distinguish a root-cap and digestive sac, | 
and who reports that the endodermis of the stem, if present, could | 
not be determined. In the present paper I shall use the term 
“root-cap ” for the entire covering of the root apex (the outer part | 
of which functions as a “digestive sac” in passing through the. 
stem), without regard to the distinction drawn by Van Tieghem || 
and Douliot. | 
The piliferous layer is marked out extremely early. In the | 
youngest roots, which have not yet emerged from the stem | 
tissues, it is visible as a columnar layer rich in contents and with |) 
large nuclei, The same is true of the apical region of the long / 
roots. Near the root-tip, while it is still enclosed in the root-cap, ) 
the future root-hair cells are already marked out by their very) 
large size and relatively gigantic nuclei (PI. VIII, Fig. 3). Before : 
these cells begin to protrude outwards to form hairs they become ( 
considerably enlarged on the inner side, displacing the cells of the | 
layer internal to them. The cells of this layer divide more fre- | 
quently than the rest of the cortex, with the result that the base ( 
of the root-hair cell becomes enclosed in what may be described i 
as a jacket of small cells (7, Pl. VIII, Figs. 3 and 4). if 
In the mature root the cortex is sharply separated into two: 
regions, an inner region in which the cells are radially arranged 
and an outer region in which the cells, which are larger, are «| 
* Van Tieghem, Ph. and Douliot, H., ‘‘Recherches comparatives sur l’origine |} 
des membres endogénes dans les plantes vasculaires,” Ann. des Sci. nat., 7 set. 
Bot. T. 8, pp. 337, 338 and Pl. 36, Figs. 557—560, 1888. 
+ Scott, D. G., ““The Apical Meristems of the Roots of Certain Aquatic || 
Monocotyledons, > New Phyt., vol. v. p. 119, 1906. 1, 
