APPENDAGES OP THE AXOPHYTE. 37 



two sponges, saturated with water, brought near each other, 

 and suspended in the air by means of a double thread. When 

 germination was sufficiently advanced, the radicles instead of 

 bearing to the right and left towards the water in the sponges', 

 glided between them, so that they ultimately hung below them 

 into the atmosphere. It is not then the humidity in the soil 

 which causes the radicles to penetrate its surface. It would 

 rather seem that this tendency ought to be attributed to a 

 particular force, which developes a sort of polarity at the 

 period of germination, which produces an opposition of growth 

 in the two extremities of the vegetable embryo, causing the 

 plumule to rise towards the zenith, and the radicle to move in 

 the direction of the earth's centre. 



The radicle fibres generally spring from the subterranean 

 portions of the axophyte, but the aerial portions of that organ 

 are equally capable of emitting them. When this is the case 

 they are designated under the name of aerial or adventitious 

 roots. Some woody vines, as the Bignonia or Trumpet-creeper, 

 the Rhus toxicodendron or Poison ivy, and the Hedera helix 

 or European ivy, climb by aerial rootlets, in which way they 

 reach the summits of the tallest trees, and loftiest buildings, 

 giving beauty even to the mouldering ruin. Such plants, 

 however, derive their nutriment from their ordinary roots 

 embedded in the soil, their copious aerial rootlets merely serv- 

 ing them for mechanical support. The tenacity with which 

 these aerial rootlets adhere to trees, rocks, and even to the 

 hardest flint, is truly astonishing ; and the height to which the 

 plants themselves will ascend, seems to cease only because 

 they can find nothing higher on which they can support them- 

 selves. In warm climates these twining plants (lianas) take 

 a much higher degree of development ; their stems are ligneous, 



4* 



