June 1, 1895.] 



KNOWLEDGE. 



14 L 



Fig. 7. — Transverse section of half of leaf of 

 Azalea procumhens. st., stomata ; A, cuticular 

 processes — " hairs " of epidermal cells. 



hairs, or hair-like structures, have evidently arisen for this 

 purpose. The ordinarj' Hawlcweed (HierKcium }}iIosella) 

 affords us a good example of this. It is a plant belonging 

 to the natural order CompositiP (the daisy order), and 

 resembles a dandelion in some respects. It is widely 

 distributed in liritain. The under surface of its leaves 

 is covered with hairs that have lost their protoplasmic 

 contents, and whose cavities are filled with air. Air, as 

 is well known, is a bad conductor of heat. The greater 

 the heat the greater tbe transpiration, and conversely a 

 decrease in temperature involves a corresponding decrease 

 in evaporation, other conditions remaining the same. Thus 

 the hairs of this plant form an effective screen from the solar 



rays, and lessen 

 the amount 

 of transpiration. 

 That this is the 

 function of these 

 structures is 

 fairly apparent. 

 The ground in 

 which the plant 

 grows frequently 

 dries up, and 

 then it is found 

 that the leaves 

 curl upwards, 

 and so expose 

 their felted under 

 surface to the 

 Sim, thus effectively lessening the chance of desiccation. 



A still better example of a plant provided with protective 

 hairs is the Edelweiss ((riutpJialixin leontopoilium), which 

 grows in the Alps at a height of from five thousand to six 

 thousand feet. Its leaves appear like tufts of wool, so 

 densely are they clothed on both surfaces with long multi- 

 cellular hairs, filled with air. The cuticle of the epidermal 

 cells is thin, and thus affords no protection to excessive 

 transpiration. These hairs not only form an effectual 

 safeguard in this respect, but also protect the plant in a 

 very efficient manner from frost and cold. 



The epidermis of the leaf of Eleagnus is covered with 

 large stellate hairs, as is shown in Fig. 8, which have a 

 similar protective function ; but a more remarkable 

 example of a plant with leaves covered with protective 

 structures is the Bochca {Sa.rifraga) falcata, a native of 

 South Africa. The hairs of this plant assume the form of 

 enormously swollen bladders. The leaf is very succulent 

 and the ordinary epidermal cells are thin-walled. The 

 walls of these bladder-like hairs are impregnated with 

 silica, and the bladders fit into one another so as to form 

 a complete siliceous coat to the surface of the leaf. They 

 thus effectually protect the desiccation of the underlying 

 tissue. By careful focussing a distinct nucleus may be 

 observed in some of these swollen cells, implying, of 

 course, the presence of protoplasm and cell-sap, and 

 showing how effectually the flinty armour performs its 

 work. A transverse section of the epidermis from the leaf 

 of Rochea is shown in Fig. 9. 



A very effective coating of protective hairs is exhibited 

 by the male shield fern i Xephrodiwn Jili.r-mas). The young 

 leaves and the upper part of the stem are densely covered 

 by innumerable scaly hairs, sometimes termed ramenta, 

 destitute of cell-contents. These must be especially 

 serviceable in shielding the plants from the injurious 

 effects that follow sudden changes of temperature, and 

 they protect the delicate growing portion from cold and 

 excessive moisture. 

 The deposition of pollen upon the stigma is called 



pollination, and there are many contrivances to prevent 

 self-pollination, and also many others to ensure cross- 

 pollination — that is, the deposition of pollen from another 



Fia. 8.- 



■Under surface of leaf of Eleagmis, with 

 stellate hairs. 



plant (of the same species). In certain cases it has been 

 shown that the seeds produced as the result of cross- 

 fertilization are more numerous and superior to those 

 resulting from self-fertilization. Cross-pollination is often 

 efl'ected by insects, and to attract them plants may go to 

 the expense of secreting nectar. But the sweet juice 

 attracts insects that are too small to effect the work of 

 cross-pollination, as well as effective larger ones. If the 

 former succeed in obtaining the coveted secretion, the plant 

 will have spent its energies in vain. Means are, therefore, 

 taken to frustrate the attempts of these small animals 

 at visiting the flowers, and hairs are often the agents 

 thus employed. These hairs pour forth a sticky secretion, 

 in which the unwary insect becomes entrapped, and 

 meets an ignominious death. As an example of this 

 we may cite the Silcne muscipula, or Catchfly, a very 

 common British plant belonging to the natural order 

 Caryophyllacefe 

 (the pink order), 

 which has received 

 its name from this 

 circumstance. The 

 sticky hairs are 

 only developed on 

 and near the floral 

 region of the stem, 

 and offer an effec- 

 tive trap for un- 

 welcome guests. 



IV. — A fourth 

 use to which hairs 

 are subservient is 

 again connected with the pollination of flowers. In some 

 cases the hairs prevent self-pollination ; in other instances 

 they are the active agents in effecting it. The Compositfe 

 afford us very good examples of both cases. The plants 

 belonging to this order are characterized by the possession 

 of five stamens attached to the petals, whose filaments are 

 free from one another, but whose anthers are united into 

 a tube. The upper part of the style is bifurcated, but the 

 forks are at first closely apposed, and their outer surface 

 is covered with hairs. The inner surface of each fork is 

 the stigmatic surface. The upper part of the style is 

 enclosed by the anthers while the flower is young, but by 

 and by an increase in the length of the style occurs causing 

 the upper hairy portion to push its way through the anther 

 tube like a bottle brush, dislodging the contained pollen 



Fia. 9. — Transrerse section of upper part 

 of ejiidermis of Rochea (^Saxifragi) falcaia^ 

 witli bladder-shaped hairs (A), whose walls 

 are impregnated with silica; ep., ordinary 



epideiTnal cell ; si., stoma. 



