138 



KNOWLEDGE. 



[June 1, 1895. 



means complete or entirely accurate. We do not find any 

 mention of the works of Airy, Buckland, Prestwich, Cayley, 

 Andrews, Dana, Giekie, CroU, Jansseu, Mallet, Sir John 

 Franklin, Newcomb, Eowland, Angstrom, and many others, 

 though space is given to their lesser contemporaries. On the 

 other hand, in the chapter dealingwith the progress of science 

 among the Arabs, from the ninth to the fifteenth century, 

 many names unfamiliar to European readers will be found. 

 Dm'ing this period the Arabs were in the van of civilization, 

 and they really form the connecting link between the Greeks 

 and ourselves. To quote Mr. Marmery, " They improved 

 upon mathematical and astronomical knowledge ; gave us 

 algebra (solving even cubic equations), extended trigo- 

 nometry, and thus met the needs of celestial geometry. 

 But they left their mark for all times equally in medicine, 

 physics and chemistry." In another chapter, the author 

 makes a comparison between Roger Bacon and Francis 

 Bacon, and shows pretty conclusively that the latter drew 

 all his inspiration from his namesake. In our opinion, 

 this is the best chapter in the book. The Elizabethan 

 Bacon is effectually stripped of the plumes borrowed from 

 the earlier Bacon — the man of the thirteenth century. 



In a book of this kind, covering so wide a ground, mistakes 

 are inevitable. Errors certainly occur, but we do not think 

 they are so numerous as to render the book useless, though 

 they necessarily detract from its value as a work of refer- 

 ence. The subject for wonder should not be so much the 

 mistakes committed as (to use an Irishism) the mistakes 

 that might have been made, but are not. Mr. Marmery 

 essayed to accompHsh a difficult task, and the result indi- 

 cates that he was a trifle too ambitious. His selection of 

 names as representatives of modern science shows clearly 

 that his judgment of the relative importance of scientific 

 work is frequently faulty. Specialists in all branches of 

 natural knowledge may well address to him the question, 

 " Who made thee a prince and a judge over us ? " 



BOOKS RECEIVED. 



The Moon, with a map of its principal features. By Thomas 

 a^vn Elger, F.R.A.S. (PhiUp & Son.) 5s. net. 



The Mir/i-afion of British Birds. By Charles Dixou. Six maps. 

 (Chapman & Hall.) Vs. 6cl. 



Geometrical Comes. By E. S. Macaulay, M.A. (Cambridge 

 University Press.) 43. 6d. 



" Things New and Old": or, Stories from lEngJish Sistory. By 

 Arnold Forster. (Cassell & Go.) Illustrated. Is. 6d. 



Mej>ort of S. P. Langlei/, Secretary of the Smithsonian Inxtitutivn, 

 for the year ending June ZOth, 1894. 



Index to the Literature of Didymiiim, 1842-1893. By A. C. 

 Langmuir, Ph.D. 



Smithsonian Geographical TaiJes. Prepared by E. S. Woodward. 

 (Washington Smithsonian Institution.) 



The Elements of Botany. By Francis Darwin, M.A., M.B., F.K.S. 

 (Cambridge University Press.) 63. 



The Wilsonian Theory and the SionyJiurst Draicings of Sunspots. 

 By Rev. WaUer Sidgreaves, S.J. 



Crystallography : a Treatise on the Morphology of Crystals. By 

 N. Story -Maskelyne, M.A., F.E.S. (Oxford : The Clarendon Press.) 

 Illustrated. 12s. 6d. 



THE FUNCTIONS OF THE HAIRS OF PLANTS. 



By J. Pentland-Smith, M.A., B.Sc. 



THE merely descriptive botanist must exist in a 

 paradise amongst the structures with whose 

 functions we are about to concern ourselves, for 

 their variety and beauty of form is almost endless. 

 A consideration of their uses, however, and of 

 their influence on the distribution of the plauta which bear 

 them, is a, much more intellectual pleasure. 



and varied functions 



It must be confessed at the outset that, although in the 

 majority of cases the biological significance of the hairs on 

 plants is perfectly obvious, there are many instances in 

 which it is impossible to conjecture their function. 



Hairs may arise from any member of a plant — from the 

 root, stem or leaf — and occasionally their presence may 

 be observed in large internal spaces. In point of origin a 

 hair is an epidermal structure — that is, it originates from 

 a cell of the epidermis, or outer skin of the plant. The 

 hairs of the root remain as simple prolongations of 

 epidermal cells, but those found on other portions of the 

 plant may develop in various ways, either remaining aa 

 smgle cells or becoming difl'erentiated into two or more 

 cells, which occasionally display a differentiation with 

 regard to function. 



The glistening hairs forming a downy covering on the 

 leaves of certain plants owe their shiny appearance to the 

 presence of air in their cavities. In many other cases the 

 hairs of the leaf and stem at maturity contain protoplasm, 

 a nucleus, and generally a large quantity of cell-sap. 

 The root-hairs always belong to the latter category. 



It would be an impossible task to compress into the 

 limits of a short paper the many 

 performed by hairs, and the 

 endless modifications of structure 

 subservient to them. We shall 

 content ourselves, then, with a 

 five-fold physiological grouping of 

 these structures, and shall give a 

 few examples of each group. 



I. — Hairs may be regarded as 

 absorptive organs. The hairs of 

 this class always retain their 

 protoplasm, nucleus, and cell-sap. 

 The root-hairs, as we have already 

 noted, belong to it. Carbon, 

 hydrogen, nitrogen, oxygen, phos- 

 phorus, potassium, magnesium, 

 calcium, and iron are the elements 

 from which the food of a green 

 plant is produced. It normally 

 obtains them all from the soil 

 (with the exception of carbon and 

 oxygen, which it derives from the 

 air) in the form of compounds 

 called mineral or inorganic salts, 

 and water. The salts must be 

 soluble in water or dilute acids 

 to be available for the nutrition 

 of the plant, as it is only in this 

 form that the roots can absorb 

 them. In search of these they 



ramify through the soil. The general surface of the root 

 does not act as an absorbent ; the root-hairs alone perform 

 this function. They contain solutions of organic acids 

 which are denser than the solutions of inorganic salts in the 

 soU, and a current is thus set up from the outside inwards. 

 It passes cell by cell to the interior of the root, where it 

 enters the vascular bundles, which carry it up the stem to 

 the leaves (see Fig. 1). It is to be specially noted in passmg 

 that this solution is very weak — it might be compared to 

 tap-water — as we shall have to speak by and by of the aid 

 given by certain hairs to the process of getting rid of the 

 excess of water. 



The root-hairs by secreting organic acids help to dissolve, 

 and so render available as food, salts insoluble in water. 

 Roots can thus be made to trace out their course in the 

 soil, if a polished slab of marble be placed at a suitable 

 depth uadergrouud. It may be noted from Fig. 2, which 



Fig . 1 . — Transverse section 

 ot portion of root of Maize 

 {Zea miis). per., pericyle 

 (from which young roots 

 arise); end., endodermis, or 

 buudle-sheath; rase, bundles, 

 vascular bundles which con- 

 duct crude nutrient material 

 from roots to leaves. 



