November 14, 1901J 



NA TURE 



41 



neither plant nor man can build high unless he builds straight. 

 A man has a general perception of the verticalness of his body 

 and of surrounding objects, but he does not trust to this sense 

 in placing brick on brick to make a house. He uses a plumb- 

 line which tells him through his eye the precise line along which 

 he must pile his bricks. The tree has also to pile one over 

 another the cells or chambers in which its protoplasmic body 

 lives, and this too must be done along a vertical line ; but the 

 plant does it by the sensitiveness to gravity of which I have 

 spoken. 



It must be clearly understood that gravity does not act 

 directly on the growth of plants. It does not act as a magnet 

 acts on iron, or to take a better example, it does not simply 

 act as gravity acts on the plumb-line in which the string is 

 kept in a vertical straight line by the weight. It might be 

 supposed that in some occult way the stem was mechanically 

 kept straight like the string, and this indeed was the view 

 formerly held about such roots as grow straight down into the 

 earth. But it is not so; the thing is not explicable mechanically. 

 Gravitation is nothing more than a sign-post or signal to the 

 plant — a signal which the plant interprets in the way best 

 suited to its success in the struggle for life, just as what we 

 see or hear gives us signals of the changes in the exterior world by 

 which we regulate our conduct. 



Vou will say that this is hard to prove, and indeed, like other 

 biological hypotheses, it can only be shown to be true by 

 explaining a number of facts. It is interesting to try to explain 

 the facts without the assumption in question. If gravity does 

 not act indirectly as a signal it must act directly, and we must 

 find a reason why, in the case of the mustard seedling above 

 referred to, the stem has grown up and the root down. There 

 is absolutely nothing in their structure or manner of growth 

 to help us to see why this difference of behaviour under 

 identical conditions should exist. And if, instead of placing 

 the mustard seedling in the dark we had grown it near the 

 window, we should have come across another remarkable phe- 

 nomenon, namely, that the stem grows towards, the root away 

 from, the light — and this is equally inexplicable on a mechanical 

 basis. 



But it may be said that it is not fair to compare a root and 

 a stem which are structurally unlike. Let us, therefore, stick 

 to roots. When the root of a bean has grown vertically down 

 into the soil for some distance it begins to bud forth into side 

 roots. These are e.xactly like the primary root from which they 

 spring ; there is no difference in structure or in machinery of 

 growth, ^'et the secondary roots do not grow vertically down, 

 but obliquely, or in some cases horizontally. There is one 

 more striking fact about the roots of the bean. The 

 secondary, like the primary roots, give off branches, and 

 these — the tertiarie; —behave differently from both primary and 

 secondary roots. For instead of directing themselves vertically 

 or horizontally, they simply treat the force of gravity with con- 

 tempt and grow just where fancy leads them. The point on 

 which I wish to insist is that it is impossible to explain on any 

 theory of the direct action of gravity why the three orders of 

 roots have three distinct modes of growth. They may remind 

 us of three generations, grandfather, father, and son, all of one 

 blood and yet behaving towards the universe in three distinct 

 ways — a fact not unknown in human society. 



On the other hand, it would not be dilificult to show that the 

 behaviour of the three orders of roots is well suited to the plant's 

 needs, and therefore we can understand how the power of be- 

 having in three different ways to the same signal has been 

 evolved. The main root takes the shortest course to the deeper 

 layers of earth ; the four or five ranks of secondary roots divide 

 the world between them and push forth all round, keeping 

 slightly below the horizontal ; the tertiaries take it for granted 

 that their predecessors have done the usual thing and that they 

 can .satisfactorily occupy the spaces left among their elders by 

 random growth. The fact that the tertiary roots have no 

 specialised sensitiveness of gravitation shows that their unre- 

 gulated growth is good enough for the necessities of the case. 

 For among organised beings necessity is the mother of develop- 

 ment, and what their brethren of second rank have developed 

 they too could assuredly have gained. To this point of view I 

 shall return, but first I should like to give a few more instances 

 of actions carried out in response to the signal of gravity ; and 

 these examples shall be from stem-structures. 



The flower-heads of a clover (T. sublerraneum) bury them- 

 selves in the ground, thus effectually sowing their own seeds, 



NO. 1672, VOL. 65] 



and they are guided to the ground by their unusual capacity of 

 curving down and directing themselves like a primary root 

 towards the centre of the earth. 



Other flower-stalks are guided by gravitation for quite different 

 purposes. Take, for instance, a common narcissus. In the 

 young condition there is a straight sli,:' i i i >'. 



.imd 



Fig. :i.—Trlfoliin. 



dition : the upper 

 downwards. 



-heads in the Trutting con- 

 has bent sharply and is growing vertically 



with its compact pointed flower bud ; but as the flower opens 

 the stalk bends close to the top and brings the flower-tube into 

 a roughly horizontal position, where it shows off its brightly 

 coloured crown to the insects that visit it. The flowers are 

 guided to the right position by the gravitation sense, and they 

 increase or diminish the angular bend in their stalk till the right 

 position is attained, as shown in Fig. 3. The same thing 



may be easily seen in the larkspur. So long as the plant is left 

 to itself the flower-stalk remains quiescent, but if the stem is 

 displaced so that the flower makes the wrong angle with the 

 vertical, the stalk is stimulated to curve, and bends until the 

 flower is once more in its proper position. 



All these cases of plants executing certain useful curvatures 



