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Show 186 THE EFFECT OF KILLING OR CnAP. III. gravity likewise resides in the tjp; so that it is the tip which excites the adjoining parts of a horizontally extended radicle to bend towar ls the centre of the earth. SECONDARY RADICLES BECOMING VERTICALLY GEOTROPIC BY 'l'BE DE8TRUC'I'ION OR INJURY OF TilE TERMINAL PAwr OF TIIE PRIMARY H.ADICLE. Sachs has shown that the lateral or secondary radicles of the bean: and probably of other plants, arc acted on by geotropism in so peculiar a manner, that they grow out horizontally or a little inclined downwards; and he has further shown* the interesting fact, that if the end of the primary radicle be cut off, one of the nearest secondary radicles changes its nature and grows perpendicularly downwards, thus replacing the primary radicle. We repeated this experiment, and planted beans with amputated radicles in friable peat, and saw the result described by Sachs ; but generally two or three of the secondary raclicles grew perpendicularly downwards. \V e also modified the experiment, by pinching young raclicles a little way above their tips, between tho arms of a U-shaped piece of thick leaden wire. The part pinched was thus flattened, and was afterwards prevented from growing thicker. Five radicles had their ends cut off, an l served as controls or stanuards. Eight were pinched ; of these 2 were pinched too severely and their ep.cls died and dropped off ; 2 were not pinched enough and were not sensibly a:ffi cted; the remaining 4 were pinched sufficiently to check the growth of the terminal part, but did not appear otherwise injured. vVhen the U-shaped wires were removed, after an * 'Arbeiteu Bot. Institut., Wiirzburg,' Heft iv. 1874, P· 622. CHAP. III. INJURING THE PRIMARY RADICLE. 187 interval of 15 days, the part beneath the wire was found to be very thin and easily broken, whilst the part above was thickened. Now in these four cases, one or more of the secondary radicles, arising from the thickened part just above the wire, had grown perpendicularly downwards. In the best case the primary radicle (the part below the wire being 1:} inch in length) was somewhat distorted, and was not half as long as three adjoining secondary radicles, which had grown vertically, or almost vertically, down wards. Some of these secondary radicles aclherou together or had become confluent. We learn from these four cases that it is not necessary, in order that a secondary radicle should assume the nature of a primary one, that the latter should be actually amputated ; it is sufficient that the flow of sap into it should be checked, and consequently should be directed into the adjoining secondary radicles; for this seems to be t~e most obvious result of the primary radicle being pmched between the arms of a U -sha peel wire. This change in the nature of secondary radicles is cle~rly analogous, as Sachs has remarked, to that WhiC.h occurs with-the shoots of trees, when the' leading one Is destroyed and is afterwards replaced by one or ~ore of the lateral shoots ; for these now grow upright mstead of sub-horizontally. But in this latter case t~e lateral shoots are rendered apogeotropic, whereas With radicles tho lateral ones are rendered geotropic. We are naturally led to suspect that the same cause acts with shoots as with roots namely an increased flow 0 f . ' ' ~ap mto the lateral ones. We made some trials with Ab2es communis and pectinata by pinchino- with wire the le a dm' g and all the latera'l shoots excbe ptino- one. But we be 1 1' eve that they were too old when ebx peri-mented on; and some were pinched too severely, and |
