Amphicarpoea monoica.—­This plant produces long thin shoots, which twine round a support and of course circumnutate.  Early in the summer shorter shoots are produced from the lower parts of the plant, which grow perpendicularly downwards and penetrate the ground.  One of these, terminating in a minute bud, was observed to bury itself in sand to a depth of 0.2 inch in 24 h.  It was lifted up and fixed in an inclined position about 25o beneath the horizon, being feebly illuminated from above.  In this position it described two vertical ellipses in 24 h.; but on the following day, when brought into the house, it circumnutated only a very little round the same spot.  Other branches were seen to penetrate the ground, and were afterwards found running like roots beneath the surface for a length of nearly two inches, and they had grown thick.  One of these, after thus running, had emerged into the air.  How far circumnutation aids these delicate branches in entering the ground we do not know; but the reflexed hairs with which they are clothed will assist in the work.  This plant produces pods in the air, and others beneath the ground; which differ greatly in appearance.  Asa Gray says* that it is the imperfect flowers on the creeping branches near the base of the plant which produce the subterranean pods; these flowers, therefore, must bury themselves like those of Arachis.  But it may be suspected that the branches which were seen by us to penetrate the ground also produce subterranean flowers and pods.]

Diageotropism.

Besides geotropism and apogeotropism, there is, according to Frank, an allied form of movement,

* ‘Manual of the Botany of the Northern United States,’ 1856, p. 106. [page 521]

namely, “transverse-geotropism,” or diageotropism, as we may call it for the sake of matching our other terms.  Under the influence of gravitation certain parts are excited to place themselves more or less transversely to the line of its action.* We made no observations on this subject, and will here only remark that the position of the secondary radicles of various plants, which extend horizontally or are a little inclined downwards, would probably be considered by Frank as due to transverse-geotropism.  As it has been shown in Chap.  I. that the secondary radicles of Cucurbita made serpentine tracks on a smoked glass-plate, they clearly circumnutated, and there can hardly be a doubt that this holds good with other secondary radicles.  It seems therefore highly probable that they place themselves in their diageotropic position by means of modified circumnutation.

Finally, we may conclude that the three kinds of movement which have now been described and which are excited by gravitation, consist of modified circumnutation.  Different parts or organs on the same plant, and the same part in different species, are thus excited to act in a widely different manner.  We can see no reason why the attraction of gravity should directly modify the state of turgescence and subsequent growth of one part on the upper side and of another part on the lower side.  We are therefore led to infer that both geotropic, apogeotropic, and diageotropic movements, the purpose of which we can generally understand,