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,
* Elfving has lately described ('Arbeiten des Bot. Instituts in Würzburg,' B. ii. 1880, p. 489) an excellent instance of such movements in the rhizomes of certain plants. [page 522]
have been acquired for the advantage of the plant by the modification of the ever-present movement of circumnutation. This, however, implies that gravitation produces some effect on the young tissues sufficient to serve as a guide to the plant. [page 523]
CHAPTER XI.
LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.
General considerations--Vicia faba, effects of amputating the tips of the radicles--Regeneration of the tips--Effects of a short exposure of the tips to geotropic action and their subsequent amputation--Effects of amputating the tips obliquely--Effects of cauterising the tips--Effects of grease on the tips--Pisum sativum, tips of radicles cauterised transversely, and on their upper and lower sides--Phaseolus, cauterisation and grease on the tips--Gossypium--Cucurbita, tips cauterised transversely, and on their upper and lower sides--Zea, tips cauterised--Concluding remarks and summary of chapter--Advantages of the sensibility to geotropism being localised in the tips of the radicles.
CIESIELSKI states* that when the roots of Pisum, Lens and Vicia were extended horizontally with their tips cut off, they were not acted on by geotropism; but some days afterwards, when a new root-cap and vegetative point had been formed, they bent themselves perpendicularly downwards. He further states that if the tips are cut off, after the roots have been left extended horizontally for some little time, but before they have begun to bend downwards, they may be placed in any position, and yet will bend as if still acted on by geotropism; and this shows that some influence had been already transmitted to the bending part from the tip before it was amputated.