86 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 



of its green leaflets. So also does Leucohryum, javense, a species native to Java. 

 Several delicate ferns of the family of the Hymenophyllacece 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 

 papillae; 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 milKmeter 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 miUimeters, and their thickness between O'OOS m.m. 

 and 0'14 m.m. 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-like 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 cm. The stems of many 

 mosses also are wrapped in a regular felt. This property is rendered very striking 

 in the species of Barbula, Dicranuvi, 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 spiraUy 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 ceUs 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 



