A part or organ which whilst young is extremely sensitive to apogeotropism ceases to be so as it grows old; and it is remarkable, as showing the independence of this sensitiveness and of the circumnutating movement, that the latter sometimes continues for a time after all power of bending from the centre of the earth has been lost.  Thus a seedling Orange bearing only 3 young leaves, with a rather stiff stem, did not curve in the least upwards during 24 h. whilst extended horizontally; yet it circumnutated all the time over a small space.  The hypocotyl of a young seedling of Cassia tora, similarly placed, became vertical in 12 h.; that of an older seedling, 1 1/4 inch in height,

* For details see ‘The Movements and Habits of Climbing Plants,’ 1875, p. 131. [page 511]

became so in 28 h.; and that of another still older one, 1 ½ inch in height, remained horizontal during two days, but distinctly circumnutated during this whole time.

When the cotyledons of Phalaris or Avena are laid horizontally, the uppermost part first bends upwards, and then the lower part; consequently, after the lower part has become much curved upwards, the upper part is compelled to curve backwards in an opposite direction, in order to straighten itself and to stand vertically; and this subsequent straightening process is likewise due to apogeotropism.  The upper part of 8 young cotyledons of Phalaris were made rigid by being cemented to thin glass rods, so that this part could not bend in the least; nevertheless, the basal part was not prevented from curving upward.  A stem or other organ which bends upwards through apogeotropism exerts considerable force; its own weight, which has of course to be lifted, was sufficient in almost every instance to cause the part at first to bend a little downwards; but the downward course was often rendered oblique by the simultaneous circumnutating movement.  The cotyledons of Avena placed horizontally, besides lifting their own weight, were able to furrow the soft sand above them, so as to leave little crescentic open spaces on the lower sides of their bases; and this is a remarkable proof of the force exerted.

As the tips of the cotyledons of Phalaris and Avena bend upwards through the action of apogeotropism before the basal part, and as these same tips when excited by a lateral light transmit some influence to the lower part, causing it to bend, we thought that the same rule might hold good with apogeotropism.  Consequently, the tips of 7 cotyledons of Phalaris were [page 512] cut off for a length in three cases of .2 inch and in the four other cases of .14, .12, .1, and .07 inch.  But these cotyledons, after being extended horizontally, bowed themselves upwards as effectually as the unmutilated specimens in the same pots, showing that sensitiveness to gravitation is not confined to their tips.

Geotropism.

This movement is directly the reverse of apogeotropism.  Many organs bend downwards through epinasty or apheliotropism or from their own weight; but we have met with very few cases of a downward movement in sub-aërial organs due to geotropism.  We shall however, give one good instance in the following section, in the case of Trifolium subterraneum, and probably in that of Arachis hypogaea.