140 



KNOWLEDGE 



[June 1, 1893. 



the agency of the marginal hairs. This results in the 

 sweUing up of the resinous material and the absorption of 

 water by the glands. In dry weather the resin, drying 

 up, encrusts the glandular hairs, and thus prevents 

 evaporation from their surface. 



Similar provisions for the absorption of water during 

 wet weather, and its retention during dry periods, are 

 marked in the family of the Saxifrages. In Sdxif'raiin 

 (lizoon, the evergreen saxifrage, the small somewhat 

 fleshy leaves are deeply serrated, and on the upper surface 

 of each tooth there is a weU-marked depression, whose 

 base is lined with cells secreting carbonate of lime. The 

 presence of this substance gives quite a greyish-white 

 appearance to the leaf margin. The incrustation is held 

 in its place by knobbed hairs that arise from certain of 

 the epidermal cells. This saxifrage, in common with its 

 allies, is found in rocky situations, where, subjected to the 

 action of a strong sun, it stands much chance of desicca- 

 tion. The incrustation of lime serves to prevent the 

 evaporation of water, but when rain comes it soaks under 

 the crust, and the delicate walled secreting cells can now 

 act as absorbents. In dry weather the crust sinks down 

 again, and plugs up the cavity ; but it often happens that, 

 even with these provisions, the leaf becomes so dried up 

 that it curls upwards, thus cracking the lime stoppers, 

 which would be easily blown away by a strong wind were 

 it not that each is held in its place by the hau's whose 

 presence we have already mentioned. 



The stem and leaves of the Pelargoniums (the 

 " Geraniums " cultivated in our glass-houses are really 

 Pelargoniums) bear numerous unicellular and multicellular 



hairs. Two forms of these 

 are represented in Fig. 4. 

 In many cases the latter 

 are more swollen at the 

 tip than the specimen to 

 the left of the figure, and 

 they then receive the name 

 of capitate hairs. The 

 outer wall of the apical cell 

 is very thick, but in wet 

 weather it swells up and 

 becomes converted into a 

 resinous material, which 

 peels oif, and permits of 

 the passage of water into 

 the interior of the cell. 

 The resinous matter is 

 secreted by the cell, and is 

 boimded on the outside by 

 a thick layer of cuticular 

 substance, which peels off with it. Then the cell secretes 

 a fresh supply of resinous substance, that hardens as the 

 weather becomes dry, and so prevents evaporation from 

 the outer surface of the hairs. 



II.— Hairs also function as maintainers of a free passage 

 for the flow of water vapom- from the stomata. The very 

 dilute solution of nutrient salts absorbed by the root-hairs 

 passes up the stem chiefly through the special conducting 

 layers of tissue called the vascular bundles, whose termina- 

 tions are the veins of the leaves. From the leaves the 

 superfluous water is evaporated, not from the general 

 surface, for that, as we have seen, is more or less impervious 

 to water, but by specially modified organs called stomata 

 (Greek crrcu^a, a mouth). If the evaporation of water be 

 retarded, it stands to reason that the plant will receive 

 less nourishment than when the process is being actively 

 carried on. Any obstacle, then, that prevents the free 

 transpiration of water will be detrimental to its growth. 



Fig. 4.^Portion of epidermis and 

 underWiiig tissue of Pelargonium, 

 ivitli secreting hair (c), and a sharp- 

 pointed hair. 



Fig. 5. — Transverse section 

 of portion of leaf of Oleander 

 {Nerinm oleander), with pit 

 on lower surface, ii., stomata; 

 A, hairs. 



Thus, any contrivance which prevents the blocking up of 

 the stomata, by water or other substances, will be more or 

 less efi'ective in promoting the 

 development of the organism. 

 The leaves of Xcrium oleander 

 (the Oleander), a plant com- 

 monly grown in our glass- 

 houses, and much used during 

 summer for outdoor decoration 

 on the Continent, have their 

 stomata situated at the base of 

 deep pits in the under epidermis. 

 The entrance to these is guarded 

 by a large number of compara- 

 tively long unicellular hairs, 

 that arise from the sides of 

 the cavities. A transverse 

 section of a portion of an 

 Oleander leaf is represented in 

 Fig. 5 ; St. are the stomata, 

 and /( the hairs ; ep. is the 

 epidermis of the under surface. 

 A surface view of a portion 

 of the epiderrnis is shown 

 in Fig. t>, the lettering being 

 the same as for the previous 

 tigure. The hairs are not 

 wetted by moisture. They 

 prevent water which may pass 



to the under surface of the leaf from blocking up the 

 stomatic orifices. As the Oleander grows in damp situa- 

 tions, transpiration, and consequently growth, would be 

 retarded were there not some means of preventing the 

 excessive moisture to which it is subjected in its native 

 habitat (the banks of streams and similai-ly moist 

 situations in Southern Europe) from blocking up the 

 stomatic orifices by drops of water. 



This is exemplified in a still more remarkable manner 

 by the rolled leaves of Azalea prucuDibens, and Kerner 

 states that it is common to rolled leaves from all parts 



of the world. In Auilea pro- 

 cumhens, the trailing Azalea, 

 the leaf is rolled inwards, as 

 shown in Fig. 7. There is 

 a marked ridge of tissue below 

 the vascular bundle of the 

 midrib, dividing the under sur- 

 face of the leaf into two well- 

 defined longitudinal grooves. 

 From the under surface of 

 the epidermal cells depend fila- 

 mentous processes of the cuticle, 

 which , perhaips , strictly speaking, 

 we should not term true hairs, 

 as they are solid, not hollow. 

 This species of azalea grows 

 in situations where the under- 

 lying soil is moist for the greater 

 part of the year, and thus renders transpiration from the 

 leaves a matter of difficulty. Hence it is necessary that, 

 when bright sunny weather does make its appearance, 

 the plant should be able to take full advantage of it. This 

 is eft'ectually provided for by the arrangement in question. 

 Moisture does not wet the hair-like cuticular processes, 

 and so the passage of water into the stomata is prevented, 

 and a clear air space is maintained around the stomatic 

 orifices. 



HI. — A third function performed by hairs is that of 

 protection. There are innumerable instances in which 



Fio. 6. — Surface Tieiv of 

 pit, with hairs and stomata 

 of the foregoing. 



