236 



KNOWLEDGE. 



[October 1, 1900. 



duce them in other ways than by the absorjDtion of food. 

 These two characters possessed by most roots — viz., a 

 tendency to grow (1) in the direction of gravity and 

 (2) away from the light — cause them to penetrate the 

 soil in which their quest (mineral food) is to be found. 



The soil varies considerably in composition, and even 

 within small areas the distribution of moisture — and 

 therefore of available plant-food — is by no means 

 uniform. Roots have a third character, which enables 

 them to follow such a course in the soil that they tap 

 the places most richly supplied with moisture and nutri- 

 ment. The root-tip is extremely sensitive to moisture, 

 and will travel through the moister pai-ts of the soil 

 even if by so doing it leaves the more direct downwai'd 

 course dictated by gravity. This fact is of immense 

 importance to the root in its search for food, for it is 

 guided, as by an unerring instinct, to just those places 

 where food in an available form is to be found. It is 

 not difficult to see for oneself evidence of this interest- 

 ing behaviour of the root in the presence of moisture. 

 A sieve is made by fastening some netting to a box from 

 which the bottom has been removed; it should be about 

 two inches deep and eight inches from side to side. 

 The sieve is filled with moist sawdust in which some 

 barley grains are sown, and then hung up in a green- 

 house* in such a way that the netted surface makes an 

 angle of 45° to 50° with the vertical. As the roots 

 grow through the bottom of the sieve the attraction 

 exercised by the moisture in the sawdust overcomes the 

 effects of gravity and they are deflected from the vertical 

 and grow downwards along the inclined surface of the 

 sieve. 



The younger parts of the root are provided with 

 so-called " root-hairs," the organs actually concerned in 

 bringing into the plant the mineral solution of the soil. 

 Each hair is produced by the elongation of a single cell of 

 the root surface.! They appear on the young part of each 

 branch of the root a little distance behind the apex, 

 and fall off as it becomes older and thicker. Only 

 those parts of the root which are provided with " hairs " 

 are able to take in solutions from without. They are 

 usually thickly placed on that part of the root where 

 they occur, and very short — as a rule, much less than 

 ^ of an inch long, but sometimes ^ inch or more. Their 

 formation is much influenced by the conditions under 

 which the root grows, for they are more abundantly 

 developed in a moderately dry than in a very wet soil. 

 On the other hand, if the soil be very diy, or so com- 

 pact and hard as to offer much resistance to the growth 

 of the root, their formation is hindered. The connection 

 between the root>hairs and the soil particles is very 

 intimate ; this is seen on uprooting almost any young 

 plant, when it will be found very diff.cult to remove 

 the last traces of soil from the younger parts of the root. 

 The root^halr, choosing in its growth the path of least 

 resistance, comes into close contact with particles of 

 soil at various points; at the places of contact its 

 cellulose wall becomes soft and jelly-like, so that the 

 particle is more or less embedded in it. J 



The root-hair is merely a long cell, and, like other 

 living cells, g is surrounded by a cellulose wall, on the 



• Or in a dark room ; in tliis case the lloor of thf room should be 

 occasionally watered, that the air may not become so dry as to cause 

 tlic withering of the roots. 



t See figure in Knowledge, July, 1!«l(l, p. Kid, /i, //. 



X See fi|,'urc in Knowlbdoe, July, 1900, p. 160, s, h 



§ Knowleuqk, January, 1900, \}. 3. 



inside of which is a lining of semi-fluid protoplasm. 

 As much of the cell as is not occupied by protoplasm 

 is filled by the cell-sap, a solution of organic and in- 

 organic substances. Outside, the root^hairs are bathed 

 by a weak solution of mineral salts, the water of the 

 soil. There are thus two solutions sepai-ated from one 

 another by a double wall, the outer of cellulose and the 

 inner of protoplasm. 



The cellidose wall being very minutely perforated,]] the 

 two solutions are provided with passages by which they 

 can communicate. Under these circumstances physical 

 forces are called into play, and the fluid particles in 

 the minute passages in the cell-wall are set in motion. 

 Owing to the presence in the cell-sap of various organic 

 substances iu the plantrcell, the solution in the soil 

 passes into the cell, which becomes in time full and the 

 elastic cellulose wall distended — a condition in which 

 the cell is called " turgid." The pressure within the 

 cell, which may be as much as four or five atmospheres, 

 is then only relieved by the transference of some of the 

 cell-sap onwards to other cells. The root-hair thus 

 obtains the nutrient solution which it seeks by the 

 purely physical process of osmosis, and passes it on to 

 the interior cells. 



If the wall separating the cell-sap and the soil 

 solution consisted of the cell-wall only, without the 

 lining of protoplasm, the passage of solution into the 

 cell would take place in the same manner, but certain 

 modifications are caused by the presence of the inner 

 wall of living protoplasm. If a slice of fresh beet-root be 

 carefully washed and placed in clean cold water no red 

 colouring matter escapes, but the water remains clear 

 and colourless. On heating, a change occurs ; when the 

 water is boiled it is coloured red by the sap which 

 escapes from the cells of the beet^root. As long 

 as the protoplasm lining the cell-wall is living it 

 prevents the coloured sap — and other substances 

 as well — from escaping, but as soon as it is 

 killed, by raising the temperature or in other ways, 

 it loses this power and the sap diffuses through it into 

 the water. The living protoplasm thus exercises some, 

 control over the dissolved substances which leave and 

 enter the cell ; and, further, a solution which enters 

 at one time is unable to escape at another. It is 

 probable that the constitution of the protoplasm varies 

 from time to time, though in what precise manner and 

 under the influence of what cause or causes it does not 

 al present admit of explanation. The fact remains that 

 the protoplasm as long as it is alive prevents some sub- 

 stances from entering the cell and others from leaving 

 it. Despite this control, a large quantity of mineral 

 matter enters the plant which is of no use to it as food 

 and may indeed be harmful. We have noticed in the 

 case of SilicaU an instance of the absorption of large 

 quantities of a substance which is of no food-value to 

 the plant. 



The roots of many plants not only absorb mineral 

 food from the soil but are also manufactories in which 

 the transformation of free Nitrogen into an oxidised 

 state occurs. It has long been known that agricultural 

 land is improved by ploughing into it the remains of 

 previous crops, and that the benefit derived is greater 

 in some cases than in others. Nearly 300 years ago 

 Bacon wrote** : — " The Fourth Helpe of Ground is the 



II Knowledoe, January, 1900, p. 3. 

 ^ Knowledge, M.ay, 1900, p. 101. 

 ** " Sylva Sylvarum," p. 146. 



