390 



NATURE 



[March 14, 1878 



described in a letter to me how these twisting grass seeds bury 

 themselves in the extremely hard and dry soil of Brazil, and are 

 thus no doubt enabled to germinate. Unfortunately these boring 

 grass-seeds do not always confine themselves to penetrating the 

 soil, but exercise their powers on both men and animals. I have 

 received accounts from India and from Italy of the way in which 

 the sharp-pointed seeds work their way through thick trousers 

 into the legs of unfortunate sportsmen. But the most extraordi- 

 nary case is that of certain grasses which work their way into 

 sheep. They often penetrate the skin deeply and in large 

 numbers, inflicting great to'tures and often causing death by 

 emaciation. Mr. Hinde, of Toronto, has given me the details of 

 this plague to sheep-farmers as it occurs in Buenos Ayres. 

 Another observer has described it in Australia.^ He states that 

 not unfrequently the seeds are found actually piercing the 

 heart, liver, and kidneys of sheep which have died from the 

 effects, I believe that the northern part of Queensland has been 

 actually given up as a sheep country because of the presence of 

 this grass. 



Another use to which locomotion is applied by animals is that 

 of finding a mate at the proper season. The curious imitation 

 of the courtship of animals which is found in Vallismeria is well 

 knovra. The stalk grows with extreme rapidity up through the 

 water till the female flower reaches the surface, and there awaits 

 the approach of the male flower, which breaks loose and floats 

 down the stream to meet her. But most plants have not even 

 this amount of locomotive power, and are therefore compelled to 

 employ either the wind or insects as go-betweens. Fortunately for 

 the beauty and sweetness of our woods and fields, insect fertilisa- 

 tion is the commonest means adopted ; and all the bright flowers 

 and sweet smells of flowers are nothing but allurements held out 

 to insects to entice them to carry the fertilising pollen from one 

 flower to another. It is curious to find a plant adopting a new 

 mode of conveying its pollen when the old one fails. Thus, a 

 wild cabbage-like plant which grows in Kerguelen's Land is now 

 fertilised by the wind, that is, it produces dry dust-like pollen, 

 which is easily carried by the wind. Now this cabbage is the 

 only species in the enormous order of the Cruciferse which is not 

 fertilised by insects ; so that we may be certain that a change has 

 taken place for which some sufficient reason must exist. And 

 the reason of the change is no doubt that the insects in Kergue- 

 len's Land are wingless, and are therefore bad distributors of 

 pollen. And to go one step further back, the reason why the 

 insects are wingless is to be found in the prevalent high winds. 

 Those insects which attempt to fly get blown out to sea, and only 

 those are preserved which are gradually giving up the habit of 

 flying. Thus the pollen of the cabbage has to learn to fly, 

 because the insects will not fly for it. 



In considering the analogies between plants and animals, one 

 cannot merely compare those functions which are strictly and 

 physiologically similar in the two kingdoms. One rather sets to 

 work and studies the needs which arise in either a plant or an 

 animal, and then discovers in what way the same need is supplied 

 in the other kingdom. There is no connection between a plant 

 having bright flowers and an animal's power of walking about, 

 yet they may, as we have seen, play the same part in the economy 

 of the two lives. 



In the life of animals the first needs that arise are supplied by 

 certain instinctive movements. The young chicken only escapes 

 from its egg by some such movements. Mr, W. Marshall has 

 also shown that the chrysalides of certain moths possess instinctive 

 movements by which they escape from the cocoon or outer case. 

 In one case a sharp spike is developed, sticking out from the 

 side of the chrysalis, and as the latter rotates the spike saws 

 the cocoon all round, so that the top lifts off like a lid. 

 Again, in young chickens Spalding has shown the existence of an 

 instinctive power of obtaining food, and instinctive recognition 

 of the hen by sound only. This was proved by a newly-hatched 

 chicken, which had never heard or seen its mother, running 

 towards a cask under which a clucking hen was hidden. The 

 powers of growth which exist in young seedlings would certainly be 

 called instinctive if they existed in animals, and they are quite 

 as indispensable as those just mentioned in supplying the wants 

 which first arise. 



These two instincts are the power of directing the growth in 

 relation to the force of gravity, and in relation to light ; the 

 first being called geotropism, the second heliotropism. As soon 

 as the young root emerges froc^ the seed-coats, it turns abruptly 

 downwards, perceiving hke the chick in what direction the earth, 

 its mother, lies. Thus the young plant fixes itself firmly in the 

 * C. Prentice, Journal of Boteiny, 1872, p. 22. 



ground as quickly as possible, and is enabled to begin to make 

 arrangements for its water supply. At the same time the young 

 stem grows upwards, and thus raises itself as much as possible 

 over its neighbours. The power of directing itself vertically 

 upwards is also a necessity to the plant, because without it no 

 massive growth like that of a tree would be possible. It would 

 be like a child trying to build a wooden house with bricks that 

 did not stand straight. Thus, both the young stem and the young 

 root have an instinctive knowledge as to where the centre of 

 the earth is — one growing towards the point, the other 

 directly away from it. This fact is so familiar to us, that 

 we faU to think of it as wonderful ; it seems a matter of 

 course, like a stone falling or a cork floating on water. 

 Yet we cannot even generalise the fact so far as to say it is the 

 nature of all stems to grow up, and all roots to grow down, for 

 some stems, such as the runner of a strawberry, have a strong 

 wish to grow down instead of up, and side roots that spring from 

 the main ones, though their method of growth is identical with 

 that of the main roots, have no wish to grow downwards. We 

 can find no structural reason at all why a root should grow down 

 and a stem up. But we can see that if a plant took to burying 

 its leaves and rearing its roots into the air, it would have a bad 

 chance in the struggle for life. It is, in fact, the needs of exist- 

 ence which have impressed these modes of growth on the different 

 organs of the plant in accordance with their various requirements. 

 On the other hand, the plant is not absolutely tied down by 

 geotropism, it is not bound to grow always in a vertical line, but 

 is ready to be turned from its course if some other direction 

 can be shown to be more advantageous. Thus Sachs ^ planted 

 peas in a little sieve, and as the roots emerged underneath, they 

 were enticed from the vertical by an oblique damp surface. 

 This power of giving up the line of growth for the sake of a 

 more advantageous position, must be of great service to roots, by 

 enabling them to choose out damp places in the earth which 

 a blind adherence to rule would have caused them to pass by. 



The other tendency, which may be also compared to an instinct, 

 is the power possessed by the growing parts of plants of perceiving 

 the position of the chief source of light. This tendency of course 

 interferes with the geo'ropic tendency, for if the tip of agrowii^ 

 shoot bends towards the light it deviates from its vertical course. 

 This contest between two instincts is well shown by placing a 

 pot of seedlings close to a lamp or a window, in which case 

 the heliotropic beats the geotropic tendency and the young 

 plants curve strongly to the light ; now if the pot is removed to 

 a dark room the geotropic tendency reasserts itself, and the seed- 

 lings become once more upright. One might fancy from this 

 that the darkness of night would be always undoing any 

 good gained by heliotropic growth in the day. An imaginary 

 case in the life of a seedling will show that it is not so. A seed- 

 ling germinates under a pile of sticks : having few competitors it 

 makes a good start, but in consequence of the darkness it begins 

 to starve as soon as it has exhausted the supply of food given it 

 by its mother plant stored up in the seed from which it 

 sprang. It starves because it is dark under the pile of sticks, 

 and without light it cannot decompose the carbonic acid of 

 the air and make starch ; carbonic acid may be said to be 

 the raw material from which a plant makes its food — but 

 without the help of light the plant is powerless to make food — 

 it starves in the midst of plenty. So that the power of know- 

 ing where the light is and of moving towards it may be just as 

 necessary to prevent a young plant starving as the power of 

 knowing a grain of corn when it sees one and of snapping it up 

 are to a young chicken. Luckily for our imaginary plant a ray 

 of light streams in between two sticks — if the plant msisted on 

 growing straight up in obedience to the geotropic instinct it 

 would lose its chance of life. Fortunately the other light-seeking 

 instinct wins the day and the plant thrusts its summit between 

 the sticks and reaches the light. And now it is clear that when 

 the plant has once get between the sticks the tendency to 

 straighten again in the night will not be able to undo the 

 advantage gained in the day by heliotropism. Besides the 

 tendency to seek the light, there is in some plants another 

 exactly opposite tendency to grow away from it. Just as 

 in the case of geotropism no reason can be given why two 

 organs should be affected in exactly an opposite manner by the 

 same cause ; no difference of structure can be perceived and no 

 difference in manner of growth can be found between a tendril 

 which grows away from the light and one which grows towards 

 it. The convenience of the plant seems to dictate the result. 

 Thus the virginian creeper climbs by forming little sticky feet at 

 » " Text Book of Botany," Eng. Tr. p. 764. 



