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Show CIRCUMNJ]TATING MOVEMENTS OF CllAJ>. TT. some cause thickened-in several instances apparently ]n correlation with the fleshy nature of the mature plant-so as to contain a store of nutriment sufficient for the seedlino-, and then that one or both cotyledons, from being su~1erfluons, decreased in sil!;e. It is. not surprising that one cotyledon a.lone sho~ld sometimes have been thus affected, for with certmn plants, for instance the cabbaO'e, the cotyledons arc at first of unequal size, owing bapparently to the manner in which they are packed within the seed. It does not, however, follow from the above connection, that w henevrr a bulb is formed at an early age, one or both cotyledons will necessarily become superfluous, and consequently more or less rudimentary. Finally, these cases offer a good illustration of the principle of compensation or balancement of growth, or, as G oothe t·xpresse~ it, " in order to spend on one side, Nature is forced to economise on the other side." Oircurnnutation and other movements of Hypocotyls and Epicotyls, whilst still arched and buried bene.ath the ground, and whilst b1·eaking through it.-Accordmg to the position in which a seed may chance to have been buried, the arched hypocotyl or epicotyl will begin to protrude in a horizontal, a more or less inclined, or in a vertical plane. Except when already standing vertically upwards, both legs of the ~:trch are acted on from the earliest period by apegeotropism. Consequently they both bend upwards, until the arch becomes vertical. During the whole of this process, even before the arch has broken through the ground, it is continually trying to circumnutate to a slight extent; as it likewise does if it happens at first to stand vertically up,-all which cases ha:c h(•cn observed and des,eribed, more or less fully, m the last chapter. After the arch has grown to some CnAP. II. HYPOCOTYLS, ETC., WHILS'r ARCHED. . 99 height upwards, the basal part ceases to circumnutate, whilst the upper part continues to do so. That an arched hypocotyl or epicotyl, with the two legs fixed in the ground, should be able to circumnutate, seemed to us, until we had read Prof. Wiesner's observations, an inexplicable fact. He has shown* in the case of certain seedlings, whose tips are bent downwards (or which nutate), that whilst the posterior side of the upper or depe11dent portion grows quickest, the anterior and opposite side of the basal portion of the same internode grows quickest; these two portions being separated by an indifferent zone, where the growth is equal on all sides. There may even be more than one indifferent zone in tho same internode; and the opposite sides of the parts above and below each such zone grow quickest. This peen· liar manner of growth is called by vViesner "undulatory nutation." Oircumnutation depends on one side of an organ gi:owing quickest (probably preceded by increased turgescence), and then another side, generally almost the opposite one, growing quickest. Now if we look at an arch like this n and suppose the whole of one side-we will say the whole convex side of both legs-to increase in length, this would not cause the arch to bend to either side. But if the ?uter side or surface of the left leg were to increase m length the arch would be pushed over to the right, a~d this would be aided by the inner side of the nght leg increasing in length. If afterwards the process were reversed, the arch would be pushed over to the opposite or left side, and so on alternately,that is, it would circumnutate. As an arched hypo:- d • 'Die undulirende Nutation Also published separately, s~.:c er Internodien,' Alcad. der Wis- p. 32. sench. (Vienna), Jan. 17th, 1878. }l 2 |