June 1, 1895.] 



KNOWLEDGE. 



139 



Fig. 2. — Lower por- 

 tion of stem and 

 "rhizoids" of Funaria 

 liiigvometrica. iVotice 

 the soil partieles cling- 

 ing to the hairs. 



is a semi-diagrammatic representation of the " rhizoids " 

 and part of the stem of a very common moss (Funaria 

 hyi/rometrica), that particles of soil are adherent to the 

 large rhizoids (root-hairs). As the specimen from which 

 this was drawn was washed repeatedly in water and 

 .brushed with a camel's-hair brush, it is evident that the 

 hairs of the plant clmg firmly to the 

 soil particles. In addition, then, to 

 absorbing water, they fasten the 

 plant in the soil. The attempt to 

 clean thoroughly the roots of a maize 

 seedling will make this obvious to 

 the reader. To obtain a specimen of 

 it without injury to the root-hairs, it 

 is necessary to germinate the fruits 

 in water or damp sawdust. 



Negative evidence in support of the 

 absorptive function of root-hairs can 

 be obtained by the examination of 

 the roots of a water plant, such as 

 Potamogeton, or the aerial roots of 

 an epiphyte, such as an orchid. In 

 the former there is no necessity for 

 such organs, as the whole root is 

 surrounded by water, and in the 

 latter there is a special modification 

 of the cortical and epidermal cells, 

 enabling these plants to condense and 

 absorb the moisture of the air. In 

 the case of a few epiphytes, however, 

 the presence of root-hairs must be 

 noted, but the plants bearing them grow in damp, shady 

 situations, where there is no chance of these dehcate 

 organs being dried up. 



Root-hairs are short-lived. As the roots increase in 

 length, new root-hairs are developed at a short distance 

 behind the growing point, and the old ones die off. The 

 plant is thus continually brought into contact with fresh 

 sources of food. 



In higher plants the absorption of water by the stem 

 and leaves is the exception, not the rule. The outer 

 layers of the epidermal cells are, moreover, less imperrious 

 to water, being infiltrated with a substance called cutin, or 

 covered with a coating of wax, which renders them quite 

 impervious. We shall now cite a few instances in which 

 the epidermal cells are modified into hairs that act un- 

 doubtedly as absorptive organs. One of the most curious 

 is found in the common ash. The upper surface of the 

 midrib of the leaf is deeply grooved, the lips of the groove 

 meeting above except at the points of origin of the leaflets. 

 Water falling on the leaflets runs into the tube at these 

 points, and once entered, its evaporation is prevented. 

 Prom the under surface and sides of the tube arise multi- 

 cellular hairs. When the transpiration of water from the 

 leaf exceeds the supply brought up from the roots, these 

 hairs absorb the water in the tube. Thus there is a guard 

 against desiccation of the leaf. 



The common chickweed {Stellaria mfdia) is an annual 

 plant that abounds as a weed in cultivated places. Its 

 native habitat is the banks of streams, and road sides. It 

 is quite devoid of hairs excepting a ridge down one side, 

 between the pairs of opposite leaves, as seen in Fig. 3. Long 

 hairs are also borne on the margins of the petioles or leaf- 

 stalks. After a shower of rain the surface of the leaves is 

 quite dry, but the water is retained by the ridge of hairs. 

 Let us consider more fully the action of these bodies. If 

 the supply of water is in excess of what can be retained by 

 the hairs of the uppermost ridge, it flows to the pair of 

 opposite leaves situated immediately below, where a certain 



amount is held by the cUiate hahs of the petioles, while 

 the superfluous supply flows down to those on the ridge 

 below, and so on. A quantity of water thus bathes the 

 bases of the hairs on the stem and leaves. The hairs are 

 multicelliTlar filaments, with nucleus, protoplasm, and cell- 

 sap. The upper cells, according to Kerner, do not absorb 

 water, but it passes through the basal cell-walls, and is 

 absorbed by the protoplasm and passed on to the interior 

 of the stem or leaf. As proof of this, he instances the fact 

 that when the water has dried ofl' the ridges the cell-walls 

 of the basal cells exhibit a striated appearance, which can 

 only be accoimted for by supposing a loss of water from 

 their interior. A good absorber is a good transpirer, unless 

 special provisions are made to prevent transpiration. 

 These basal cells, then, having no special protection, 

 exhale in dry weather the water they previously absorbed 

 in wet. It is somewhat diflicult to account for the use of 

 the extra supply of water in this case. Many plants 

 growing by the banks of streams are likewise endowed. 

 In damp situations it would appear at first sight that such 

 a supply is superfluous. Perhaps Kerner's explanation, 

 put forward for similar cases, and which seems to be the 

 only one forthcoming, is the correct one. Nitrogen is one 

 of the main constituents of plant food. As we have stated, 

 a green plant normally obtains it from the soil in the form of 

 soluble inorganic salts. This constituent of food is lacking 

 in sufiicient quantities in marshy situations, hence we find 

 plants like Drosera (sundew) and Pinguicula (Butterwort) 

 inhabiting these regions, endowed with capturing and 

 digestive apparatus enabling them to obtain nitrogen from 

 the bodies of living animals, which they retain, kill, and 

 then digest. Certain members of the pea tribe are able to 

 make use of the 

 nitrogen of the air 

 (see March and April 

 numbers of Know- 

 ledge, 1894) as a 

 source of food, but 

 it has never been 

 contended that there 

 is any reason to 

 suppose that other 

 higher plants are 

 likewise endowed. 

 In the air, however, 

 ammonia and nitric 

 acid are present 

 in very small quan- 

 tities. They are 

 carried to the soil by 



rain water, and it has been estimated that from four to 

 twenty pounds of nitrogen in the form of nitric acid may 

 be distributed annually over an acre of gi-oimd by the 

 rainfall. May it not be the case, then, that the tiny 

 droplets of water retained by the leaves of this plant after 

 every shower of rain contain in the aggregate a supply of 

 nitrogen sufficient to make up for the deficiency in the soil, 

 which, being absorbed in the mamier indicated, materially 

 benefit the plant ? 



A very pretty contrivance for absorption and retention 

 of water is seen on the under surface of the leaves of the 

 Alpine rose (Rhudodendron hirsutum). It consists of a 

 number of multicellular hairs, each shaped like a disc, 

 supported by a short stalk. Each hair is lodged in a 

 depression on the under surface of the leaf, generally 

 situated below a vein. The cells of the glandular hairs 

 secrete a resinous matter that forms a brown incrustation 

 on the under surface of the leaf. When rain falls on the 

 upper surface it soon reaches the under surface, partly by 



Fig. 3. — Portion of stem of Stellaria 



media (chickweed), enlarged. Notice 



the unilateral ridges of hairs, and the 



hau's on the petioles. 



