THE ROOTS OF PLANTS. 



17 



sign of gi'owtli. The roots of certain plants ap- 

 pear to excrete a peculiar matter, which varies 

 in the different species. Du Ilamel mentions, 

 that having caused some old elms to be rooted 

 up, he found the earth about their roots of a 

 darker and more unctuous colour than that 

 around. This unctuous fatty matter was pro- 

 duced by excretion from the roots. To this 

 matter, which varies, as we have said, in dif- 

 ferent species of plants, the sympathies and 

 antipathies which certain vegetables have for 

 each other is no doubt to be attributed. For it 

 is well known, that ctrtain plants in a manner 

 seek one another, and live constantly near each 

 other. Such are called social plants; while, on 

 the contrary, others seem hurt by these peculiar 

 matters, and will not grow neai\ Hence, too, the 

 well known fact, that cei-tain vegetables will 

 not tlirive if successively planted in the same 

 soil. It has been remarked, that roots have a 

 marked tendency to grow in tha direction of 

 veins of good soil; and that they are often ex- 

 tended considerably, in order to reach the places 

 where the soil is richer, and more friable. They 

 then grow with more vigour and rapidity. Du 

 Hamel states, that wisliing to protect a field of 

 excellent soil from the roots of a row of elms 

 which were extending in that direction, and 

 wasting a part of it, he caused a deep trench to 

 be sunk along the row of trees, which cut across 

 all the roots that stretched into the field. But 

 soon after, the new roots, on aniving at one of 

 the sides i<f the ditch, curved downwards, follow- 

 ing the slope until they amved at its lower part, 

 when they, proceeding horizontally under the 

 ditch, rose again on the other side, following the 

 opposite slope, and extended anew into the field. 

 The roots of trees have not all the same facility 

 of penetrating the hard subsoil. Du Hamel ob- 

 served that a vine root had penetrated a very 

 hard subsoil to a great depth, while an elm-root 

 had been stopped by it, and had in a manner 

 retraced its steps. We have already remarked 

 tliat the root has a natural and invincible ten- 

 dency to direct itself towards the centre of the 

 earth. This tendency is especially observed in 

 this part at the moment it begins to be developed 

 from the seed. It is afterwards less apparent, 

 although it always exists, especially in those 

 roots which are simple, as in the top root of 

 those which are branched, for it frequently does 

 not exist in the lateral ramifications of the root. 

 Whatever obstacles may be opposed to this na- 

 tural tendency of the radicle, it possesses the 

 power of surmounting them. Thus, if a ger- 

 minating bean or pea be placed in such a manner 

 that the seed lobes are situated in the earth, and 

 the radicle in the air, the radicle is soon seen to 

 bend towards the earth, and immerse itself in it. 

 Tliis phenomenon has given rise to much specula- 

 tiiin, and has received various exolauations. 



Some suppose that the root has a tendency to 

 descend, because the fluids which it contains are 

 less elaborated, and consequently heavier than 

 those of the stem. But this explanation is con- 

 tradicted by facts. In certain exotic vegetables, 

 such as dusiarosca, we see roots forming upon 

 the stem at a great heiglit, and descending per- 

 pendicularly to penetrate into the ground. Now, 

 in this case, the fluids contained in these aerial 

 roots are of the same nature as those which cir- 

 culate in the stem, and yet these roots, in place 

 of rising like it, descend towards the earth. It 

 is not, therefore, the difierence of the weight of 

 the fluids that gives them this tendency towards 

 the centre of the earth. Others have imagined 

 that they discovered the cause in the avidity of 

 roots for moisture, which is more abundant in 

 the earth than in the atmosphere. Du Hamel, 

 ■with the view of ascei-taining the truth of this 

 ex])lanation, made seeds germinate between tv.-o 

 moist sponges, suspended in the air. The roots, 

 in place of directing themselves towards either 

 of the two sponges, which were well soaked with 

 water, crept between them, and hung out be- 

 low; thus tending towards the earth. It is not 

 moisture, then, that attracts roots towards the 

 earth's centre, as is partly illustrated by another 

 experiment. Dutrochet fiUed a box with earth, 

 in the bottom of which several holes were bored. 

 In these holes he placed French beans in a state 

 of germination, and suspended the box in the 

 open air, at a height of about twenty feet. In 

 tliis manner, the seeds, being placed in the holes 

 formed at the lower surface of the box, received 

 from beneath the influence of the atmosphei-e 

 and light, and the moist earth was placed above 

 them. If the humid earth be the cause which 

 determines the direction of the radicle in this 

 case, it ought to be seen ascending into the earth 

 which lies above it; and the stem, on the contrary, 

 ought to descend into the atmosphere placed 

 below it. This, however, did not happen; the 

 radicles of the seeds descended into the atmos- 

 phere, where they soon perished, while the 

 plumules mounted upwards into the earth. 



Mr Knight, the celebrated botanist, wished 

 further to ascertain, by experiment, whether this 

 do\vnward tendency could be destroyed by a 

 rapid circular motion communicated to germin- 

 ating seeds. He accordingly fixed some seeds of 

 French beans in the nave of a wheel, kept con- 

 tinually moving in a vertical plane by a stream ' 

 of water, the wheel performing one hundred 

 and fifty revolutions in a minute. The seeds, 

 which were placed in some moss, kept constantly 

 moistened, soon began to germinate. All the 

 radicles were directed towards the circumference 

 of the wheel, and all the gemmules towards its 

 centre. By each of these directions, the gem- 

 mules and radicles obeyed their natural and 

 opposite tendencies. The same gentleman ma3e 



