692 



THIS TROPICAL AGRICULTURIST. 



[March 2, 1885. 



peculiarly sensitive to the contact of solid bodies. This 

 extremely curious phenomenon could only be fully de- 

 scribed by references to experiments and matters which 

 we have scant time for. It must suffice, therefore, to 

 state that there is evidence to show that the extreme tip 

 of the root, on coming in contact with a hard resistant 

 bddy, is caused to turn aside from that body, and if it 

 comes simultaneously into contact with two bodies, one 

 of which is harder than the other, it is caused to bend 

 away from the harder of the two. This property is all 

 the more curious because, at a portion of the root a 

 very short distance behind the tip, coutact with a solid 

 body causes that part of the root to curve over the 

 touc ting body, -much in the way that my finger is now 

 curved over this wooden pointer. As already stated, time 

 will not admit of our examining these very matters more 

 closely — they form subjects for lectures in themselves. 



But we have not yet finished our survey of what these 

 sensitive tips of the roots are capable of. Experiments 

 show that they turn towards a wet surface or atmo- 

 sphere—a fact of great importance, and one which no 

 doubt lies at the base of the explanation of the choking 

 up of drain-pipes, &c, by the roots of neighbouring trees. 

 Further, the apex of the root of such a plant as the 

 bean we are considering avoids the light — avoids it as 

 energetically as the leaves and green part turn to- 

 wards it. The two facts thus tersely put, viz., that the 

 tip of the root tends towards a damp spot and avoids 

 an illuminated one, are of course also in agreement with 

 the rest of the behaviour of our germinating bean, and 

 hence the root descends into the damp, moist granular 

 soil. 



It is now time to see what sort of structure this 

 wonderful root-tip possesses, and to inquire whence comes 

 the impulse which drives it forwards into the soil — for 

 it will be seen that while the forces producing the various 

 curvatures which have been referred to tend to guide the 

 apex of the root downwards between the particles of soil, 

 towards the darker, moister, deeper parts, they cannot 

 be expected to drive it into the soil. 



In the first place, the tip is a firm, conical, smooth 

 body, covered with a slippery, loose root-cap, as seen in 

 the diagrams. Now, it cannot be too carefully borne in 

 mind that the true tip of the root, beneath the cover- 

 ing cap, is resistant and somewhat elastic; it consists of 

 multitudes of minute tightly-packed cells, each densely 

 filled with protoplasmic substance containing very little 

 water, and of a consistency resembling in some degree 

 that of a well-made, hard-set jelly. Perhaps, indeed, a 

 better idea of it may be gained if the conical tip of the 

 root is compared to a firm, resistant jelly, cut up by 

 delicate partitions into multitudes of minute blocks, which, 

 however, are not separated from one another at all. In 

 any case, it is clear that such a cone, if steadily and 

 slowly driven by a persistant force from behind, is ad- 

 mirably adapted for penetrating between the particles of 

 soil, especially if we bear in mind the following facts: (,1) 

 the cone is protected by a slippery cap of loose cells, 

 which prevents the abrasions of the particles of soil from 

 injuring the cells beneath; (2) the driving force is steady 

 and continuous, and directed vertically, i e. along the axis 

 of the cone; (3) the tip oscillates slightly from side to 

 side, and is thus probably (though not to any very great 

 extent) insinuated between the earthy particles, no doubt 

 beiug aided to a certain extent by other properties to 

 which allusion has been made. It is of course obvious 

 that the last thing we should expect of such a cone is 

 that it could take up quantities of water from the soil: 

 its structure is clearly in no wayjadapted for such a pur- 

 pose, if only from the fact that there would be no- 

 where for the water to effect an entrance. 



And now comes the question, What is this steady- 

 continuous driving force from behind? Well it is due to, 

 the simultaneous elongation of the hundreds of thousands 

 of little cells situated a short distance behind the more 

 rigid cone we have just examined. No doubt it seems a 

 hard fact to grasp— that the absorption of water, and 

 the intercalation of minute particles of substance in the 

 interior T>f the cells shown in this diagram should be 

 capable of steadily driving the apex of the root into the 

 soil; but it is a fact nevertheless. Perhaps you will 

 apprehend the matter more clearly if I offer you a well- 



known illustration which, it is true, does not exactly 

 cover all the facts, but when will, at any rate, aid you 

 in overenming some initial difficulties' You are well 

 awere that a wedge of wooden driven firmly into a crack 

 in a rock and then moistened, swells, and that it may 

 swell so powerfully as to facture the rock; very well, 

 the elonnation of the cells behind, which steadily drives 

 the firm cone of the root forwards, is to a great extent 

 due to the absorption of water, which causes each cell 

 to grow longer. I say to a great extent' because, while 

 the water is, on the one hand, absorbed in a slightly 

 different way and enlarges the volume of each cell to 

 a much greater extent, there are on the other hand, forces 

 at work which cause new particles of substance to be added 

 to those originally composing the cells, and so fix the cells, 

 as it were, in their condition of greater elongation, strength- 

 ening them at the same time. But this is not all. Be- 

 sides growing longer, and thus driving the apex steadily 

 forwards, the cells behind increase in diameter, and bo 

 push aside the particles of the soil with a force which 

 would astonish you if I entered into figures ; this, how- 

 ever, can only be adverted to here, since we must now 

 pass to the explanation of one or two other points. 



It is clear that, great as the driving force supplied by 

 so many elongating cells— and, of course, it is upon the 

 simultaneous action of countless thousands of cells that 

 the driving power depends— it would soon cease to be 

 of much use unless a holdfast were insured at some 

 point behind. This brings me to the consideration of an 

 extremely important matter, and one on which I hope 

 to make you quite clear. At first, while the rcot is 

 still very young (as in this diagram), the weight of the 

 seed above, with that of any soil covering it, seems to 

 suffice to afford the necessary points of application; and 

 this will doubtless be supplemented immediately afterwards 

 by the increase in diameter of the upper part of the root. 

 When the root has attained some little length, how- 

 ever, a striking change takes place in its behaviour to the 

 surrounding soil. First, let me call your attention to the 

 following points, as illustrated by these diagrams. When 

 the young primary root has attained a length of about 

 four to six or eight inches — depending on circumstances 

 which we need not occupy time in examining — the older 

 portion nearest the seed has ceased to grow in length, 

 and its surface is becoming clothed with a dense cover- 

 ing of very delicate hairs, which will be referred to in 

 future as the "root-hairs." Each root-hair is au extremely 

 slender sac— a sort of long tubular bladder, in fact— 

 which possesses in virtue of its peculiar organisation an 

 extraordinary aptitude for taking up water, and for attach- 

 ing itself to the particles of soil which it comes in con- 

 tact. These facts are well illustrated by reference to these 

 diagrams, to which I wish your attention for a few 

 minutes. 



From the delicacy of these root-hairs, and from their 

 springing at right angles from the surface of this part 

 of the root, radiating in all directions between the particles 

 of soil, to which they immediately proceeded to glue 

 themselves, it is obvious that they are saved from being 

 torn away as the tip of the root is slowly driven for- 

 wards between the particles of soil; if they were to arise 

 on the tip itself, or on the parts which are elongating 

 behind it, they would infallibly be removed by the ab- 

 rasion or the particles of soil. Instead of this, however, 

 they become developed on the parts behind in successive 

 multitudes as those parts cease to elongate. 



At the same time, the thousands of points of attach- 

 ment established by the root-hairs afford the holdfast 

 which becomes more and more necessary as the apex of 

 the root is driven further and further forwards, and as 

 the weight of the aerial parts of the plant, with their in- 

 creasing surfaces exposed to wind and weather become 

 larger. 



Meanwhile, leaving aside for the moment the consider- 

 ation of how these millions of root-hairs take up the 

 water and food-matters from the soil, the young root has 

 been making preparations for obtaining a still firmer and 

 wider holdfast on the soil, which will, at the same time, 

 enable them to absorb water and food-materials at 

 millions of new points further and further removed from 

 the centre at which the primary root commenced its 

 operations. To understand this, I must call your atten- 



