NA TU RE 



[Dec. 25, i ! 



two. This property is all the more curious because, at a portion 

 of the root a very short distance behind the tip, contact with a 

 solid body causes that part of the root to curve over the touching 

 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 remarkable matters more closely — they 

 form subjects for lectures in themselves. 



But we have not yet finished our survey of what these sensi- 

 tive tips of the roots are capable of. Experiments show that 

 they turn towards a wet surface or atmosphere — a fact of great 

 importance, and one which no doubt lies at the base of the ex- 

 planation 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 parts turn towards 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 germi- 

 nating 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 

 whi'e 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 covering 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, abetter idea of it may 

 be gained if the conical tip of the root is compared to a firm, re- 

 sistant 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 persistent force from behind, is admirably 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) in- 

 sinuated between the earthy particles, no doubt being 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 

 frun the soil : its structure is clearly in no way adapted for such 

 a purpose, if only from the fact that there would be nowhere 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 havejust 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 of 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 which will, at any rate, 

 aid you in overcoming some initial difficulties. You are well 

 aware that a wedge of wood driven firmly into a crack in a rock 

 and then moistened, swells, and that it may swell so powerfully 

 as to fracture the rock ; very well, the elongation 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 

 w.ry 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 lie added to those originally 

 composing the cells, and s , fix the cells, as it were, in their 

 condition of greater elongation, strengthening them at the same 

 time. But this is not all. Besides growing longer, and thus 

 driving the apex steadily forwards, the cells behind increase in 



diameter, and so 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 is the driving force supplied by so 

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

 neous 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 root 

 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, however, 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 

 covering of very delicate hairs, which will be referred to in future 

 as the " root-hairs." Each root-hair is an 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 attaching itself to the particles 

 of soil with which it comes in contact. 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 proceed to glue themselves, it is obvious that they 

 are saved from being torn away as the tip of the root is slowly 

 driven forwards between the particles of soil ; if they were to 

 arise on the tip itself, or on the parts which are elongating be- 

 hind it, they would infallibly be removed by the abrasion of the 

 particles of soil. Instead of this, however, they become de- 

 veloped on the parts behind in successive multitudes as those 

 parts cease to elongate. 



At the same time, the thousands of points of attachment es- 

 tablished 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 increasing surfaces exposed to wind and 

 weather, become larger. 



Meanwhile, leaving aside for the moment the consideration of 

 how these millions of root-hairs take up the water and food- 

 matters from the soil, the young root has been making prepara- 

 1 ions fu- obtaining a still firmer and wider holdfast on the soil, 

 which will, al 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 attention 

 to this diagram, showing how the branching of the root proper 

 is brought about. In the interior of the growing root a number 

 of cells begin to multiply at certain points, and to form the 

 young beginnings of lateral roots or rootlets ; further back you 

 see these young lateral roots upheaving the tissues of their parent 

 root as minute knobs. By this time, however, these portions 

 of the mother root have ceased to grow in length, and thus the 

 lender little tips of the lateral roots can protrude and be pushed 

 into the soil around without danger of being dragged off or injured, 

 as they would inevitably be if this part of their mother root were 

 -Mil actively elongating. Notice carefully the exquisite adapta- 

 tion to the circumstances, though brought about in a slightly 

 different manner ; no time is lost in the preparation of the young 

 root branches within the tissues of the parent root, but the 

 tender tips, as in the case of the root-hairs, only proceed to 

 grow radially into the surrounding soil when the growth of the 

 mother root in a direction across their long axes has ceased. 



Time will not allow of our examining these matters more in 

 detail ; but I cannot avoid calling your attention to the fact that 

 these lateral roots are sensitive to gravitation in a manner dif- 

 ferent from the primary root — they grow, not straight down 

 towards the centre of the earth, but across the vertical, it may 

 be more or less inclined, in different cases. In other respects they 



