Dec. 25, 1884] 



NATURE 



185 



resemble the primary root generally, in their turn producing root- 

 hairs and daughter roots, which radiate from them in all direc- 

 tions into new portions of the soil, as shown in this diagram. 



I need not do more than point out to you that it would be 

 difficult to conceive of a series of adaptations better calculated 

 to insure that the various parts of the root-system come succes- 

 sively in contact with the whole mass of soil traversed ; and when 

 your eyes follow mine over this diagram, you will agree that 

 matters have become so arranged, so to speak, between the 

 roots and the soil, that every part of the latter is laid under 

 contribution. Notice how this vertical cylinder of earth is first 

 bored through by the primary root, and then traversed in all 

 directions by the root-hairs, in a wave, as it were, passing from 

 above downwards. Next come the lateral roots, burrowing in 

 all directions from the main shaft, and each in turn demanding 

 toll from the cylinder around it by means of its wave of root- 

 hairs. Then follow tunnelings along the lengths of each of 

 these rootlets, and on all sides at right angles to them, until 

 every nook anil cranny has been investigated by these enterprising 

 rootlets and their prying root-hairs. Quite apart from all else, 

 therefore, the root-system obtains a greater and greater holdfast 

 on the soil by driving its tips in on all sides. 



But I must now draw your attention to some matters which 

 throw even more light on our subject. The root-hairs, as they 

 develop successively from above downwards on the primary root, 

 or on the lateral rootlets, come into the closest contact with the 

 particles of soil — contact so close and firm, in fact, that they 

 cannot be torn away without injury. There are experiments to 

 prove that their cellulose walls become actually moulded and 

 gummed on to the solid particles of quartz, slate, and other rocks 

 of which ordinary soils are composed, and this diagram shows 

 how we can lift up a relatively large cylinder of soil adhering 

 to the root-hairs of a young seedling. 



Now you are probably aware that the sort of soil in which a 

 healthy plant flourishes contains air-bubbles as well as water in 

 the interstices between the particles, and into which the root- 

 hairs become insinuated. Bearing this in mind, you will have 

 no difficulty in understanding from the diagram how the root- 

 hairs absorb the aerated water necessary for their well-being. I 

 need simply make the additional remark that each little bag-like 

 root -hair takes up the liquid water through its permeable walls 

 into its interior, in some respects very much as a bladder full of a 

 solution of sugar or salt would absorb water if placed in it. 



Hut this water taken up by the root-hairs and passed onjinto 

 the rootlets and so on up the stem (a process for which pro- 

 visions are made which we cannot go into here), is not pure 

 water ; it contains, besides air, certain small proportions of the 

 soluble matters found in all soils. It is, in fact, much like 

 ordinary drinking-water from a well orspring, which always con- 

 tains some matters in solution. But the roots want certain other 

 minerals, which will not dissolve in pure water to a sufficient 

 extent under ordinary circumstances. Well, the root-hairs, in 

 making use of the oxygen which they, like all other living bodies, 

 require, give off small quantities of acids which aid the solution 

 of these more refractory matters. 



And now I have finished — not because the subject is exhausted, 

 but because the time at our disposal is. I hope the object has 

 been attained, and that you fully realise how well worthy of study 

 is a common living ro t. Not only is it instructive as a simple 

 object of dissection, a subject upon which I have had no time to 

 dwell, but the peculiar properties which stamp it as a living organ 

 themselves afford material for much thought and investigation. 

 When we go further, however, and see how the structure and the 

 functions depend upon one another, some very curious reflections 

 thrust themselves upon us ; and if time had allowed us to look 

 at these matters from the other platforms of view — to see how 

 old errors have gradually been explained away on the part of ob- 

 servers, and how what may be called improved adaptations haw- 

 arisen in the evolution of the root as an organ — these reflections 

 would have obtained in depth. But we have taken a glimpse at 

 matters still more comprehensive : we have touched upon that 

 important question of the relation of the root to its physical 

 environment, and it is not difficult to see numerous points where 

 the struggle must have been intense before the plastic substance 

 of the root was enabled to meet the requirements necessary before 

 it could become a dweller in the land. The evidence of progress 

 and adaptation to its environment on the part of the root is, in 

 fact, so striking and conclusive, that we might take it as a text 

 for a sermon on evolution were such necessary. I have been 

 strongly tempted to occupy some more time with reference to the 

 ■nteresting phenomena shown by roots which cling to trees and 



walls, &c, or which rob other plants of food-materials ; and had 

 time allowed, I would have liked to say a few words about 

 some other adaptations, such as those by means of which roots 

 become pulled up taut in the soil. However, these and other 

 matters cannot be even mentioned, and, indeed, each one 

 deserves a lecture to itself. 



FOCAL LINES 

 IXniEN a pencil of light proceeding from a luminous point is 

 incident upon a prism, the rays after refraction do not 

 as a rule diverge from a point, but from two short lines at 

 right angles to each other at some distance apart depending on the 

 angle of incidence of the pencil. These lines are known as the 

 focal lines of the pencil. If the edge of the prism be vertical 

 and the axis of the pencil lie in a horizontal plane, the focal 

 lines are respectively horizontal and vertical. The position of 

 the horizontal line is independent of the angle of incidence of 

 the pencil, its distance from the prism being the same as that of 

 the luminous point, or with the notation of Parkinson's " Optics " 

 (p. 88)- 



Z\ 2 — It. 



The distance from the prism of the vertical focal line is, on the 

 other hand, dependent on the angle of incidence, its position 

 being given by the formula — 



cos-*' cos 2 J/ 



v t = — Jl 1.». 



cos-* cos "1^' 



The image of an object viewed through the prism will appear 

 between the two focal lines, and will be formed by the circles of 

 least confusion. The two focal lines will coincide in position, 

 and they, and the circles of least confusion, will consequently be- 

 come points if * = *', that is, if the prism be placed in the posi- 

 tion of minimum deviation. 



All these phenomena of refraction by a prism, which are of 

 great importance to the spectroscope, may be verified in a very 

 striking manner by u-ing as an object a piece of wire gauze, 

 placed so that one set of wires is horizontal and the other 

 vertical, and illuminated by a sodium flame placed behind it. 

 If the light pass directly from the gauze to the prism, the focal 

 lines are of course virtual, but they may be easily viewed and 

 their positions identified by means of a telescope which will 

 focus an object at a short distance. For one position of 

 the eye-piece of the telescope the vertical wires are seen 

 distinctly while no horizontal wires are seen ; whereas for 

 another position the horizontal wires may be focused, but then 

 the vertical ones are no longer visible unless the prism is in the 

 position of minimum deviation. Between these two positions of 

 the eye-piece is a third, for which a blurred image of the gauze 

 is seen corresponding to the circles of least confusion. The 

 positions of the lines may be determined by ascertaining where 

 an object must be placed, when the prism is removed, so as to 

 be in focus in the telescope for the two positions of the eye-piece 

 corresponding to the two focal lines respectively. 



The experiment is, however, much more striking if the focal 

 lines be made real by interposing between the gauze and the 

 prism a convex lens of somewhat long focal length. The vertical 

 and horizontal images may then be viewed by means of an 

 ordinary watchmaker's glass, or, better still, by a telescope 

 eye-piece mounted behind a second gauze with its wires set at 

 45° to the horizon. With this arrangement the images cor- 

 responding to the two focal lines can be seen very clearly, and 

 their distances from the prism accurately measured. It is very 

 interesting to place the prism first in the position of minimum 

 deviation, and focus the magnifier upon the image of the gauze, 

 showing both horizontal and vertical wires clearly defined ; 

 then on gradually turning the prism the vertical lines disappear 

 completely, leaving a set of horizontal bars across the uniform 

 field, thus verifying the first formula cited above. 



If however, the eye-piece be drawn back some way, a badly- 

 defined image of the gauze can be obtained corresponding to the 

 circles of lea t confusion, and, on withdrawing the eye-piece still 

 further, the horizontal lines disappear entirely, while the vertical 

 lines come out sharply defined as a set of vertical bars across a 

 uniform field. As the experiment was arranged here, with a 

 prism of about 9° and the horizontal focal line about two feet 

 from the prism, the distance between the two images was fully 

 six inches when the prism was turned through an angle of about 

 1 5° from the position of minimum deviation. 



The properties of the focal lines formed by a pencil incident 



