86 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 
of its green leaflets. So also does Leucobrywm javense, a species native to Java. 
Several delicate ferns of the family of the Hymenophyllacee exhibit them on their 
subterranean stems. Many liverworts and the prothalli of ferns bear them on the 
under surfaces of their flat thalli which lie outspread on damp earth. But most 
commonly of all are they to be found close behind the growing tips of roots. Their 
form does not vary very much. On the roots of plants fringing the sources of cold 
mountain-springs, as on those of many marsh-plants in low-lying land, they are in 
the form of comparatively large, oblong, flattened, closely united cells, with thin 
walls and colourless contents. In some conifers, whilst having in the main the 
shape just described, they differ in that they are arched outwards so as to form 
papilla; but in most other phanerogams the external cell-wall projects outwards, 
and the whole absorptive cell develops into a slender tube, set perpendicularly to 
the longitudinal axis of the root (fig. 12‘). 
Seen with the naked eye, or but slightly magnified, these delicate tubes look like 
fine hairs, and have received the name of “root-hairs.” The end of a root often 
appears to be covered with velvety pile, and the absorptive cells are then very 
closely packed; more than four hundred per square millimeter have been occa- 
sionally counted. In other cases, however, there are hardly more than ten on a 
square millimeter. When in such small numbers they are usually elongated and 
clearly visible to the naked eye. Their length, for the most part, varies from the 
fraction of a millimeter to three millimeters, and their thickness between 0'008 m.m. 
and 014mm. It is only exceptionally that one meets with plants, rooted in mud, 
possessing root-hairs 5 m.m. or more in length. The absorptive cells of phanero- 
gams are almost always simple epidermal cells of the particular part of the plant 
that bears them, and are not partitioned by any transverse walls. In mosses and 
fern prothalli, on the other hand, the absorption-cells are generally segmented by 
transverse septa and are usually greatly elongated. In those liverworts which 
belong to the genus Marchantia they form a thick felt on the under side of the 
leaf-lke plant, or rather, on such part of it as is turned away from the light, and 
some of these tangled rhizoids attain a length of nearly 2 c.m. The stems of many 
mosses also are wrapped in a regular felt. This property is rendered very striking 
in the species of Barbula, Dicranwm, and Mnium, and especially in such forms as 
have bright green leaves, by the reddish-brown colour of the cells in question. 
Sometimes the long capillary cells of which the felt is composed are twisted 
together spirally like the strands of a rope. A good instance of this is Polytri- 
chum. These fine, hair-like, segmented and branched structures, found on mosses, 
variously matted and intertwisted, are called rhizoids. But only those cells which 
come into contact with the earth-particles are truly absorbent. The rest do not 
serve to imbibe from the ground, but to conduct the aqueous solution of food-salts, 
after it has been taken up by the absorptive cells, to the stem and to the leaves. 
The tubular cells resulting from the development of a root’s epidermis are placed, 
as before observed, at right angles to its longitudinal axis. They only grow, how- 
ever, in earth that is very damp, and even then their course is not always a straight 
