When the U-shaped wires were removed, after an
* 'Arbeiten Bot. Institut., Würzburg,' Heft iv. 1874, p. 622. [page 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 ½ inch in length) was somewhat distorted, and was not half as long as three adjoining secondary radicles, which had grown vertically, or almost vertically, downwards. Some of these secondary radicles adhered 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 the most obvious result of the primary radicle being pinched between the arms of a U-shaped wire.
This change in the nature of secondary radicles is clearly analogous, as Sachs has remarked, to that which occurs with the shoots of trees, when the leading one is destroyed and is afterwards replaced by one or more of the lateral shoots; for these now grow upright instead of sub-horizontally. But in this latter case the lateral shoots are rendered apogeotropic, whereas with radicles the 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 of sap into the lateral ones. We made some trials with Abies communis and pectinata, by pinching with wire the leading and all the lateral shoots excepting one. But we believe that they were too old when experimented on; and some were pinched too severely, and [page 188] some not enough. Only one case succeeded, namely, with the spruce-fir. The leading shoot was not killed, but its growth was checked; at its base there were three lateral shoots in a whorl, two of which were pinched, one being thus killed; the third was left untouched. These lateral shoots, when operated on (July 14th) stood at an angle of 8o above the horizon; by Sept. 8th the unpinched one had risen 35o; by Oct. 4th it had risen 46o, and by Jan. 26th 48o, and it had now become a little curved inwards. Part of this rise of 48o may be attributed to ordinary growth, for the pinched shoot rose 12o within the same period. It thus follows that the unpinched shoot stood, on Jan. 26th, 56o above the horizon, or 34o from the vertical; and it was thus obviously almost ready to replace the slowly growing, pinched, leading shoot. Nevertheless, we feel some doubt about this experiment, for we have since observed with spruce-firs growing rather unhealthily, that the lateral shoots near the summit sometimes become highly inclined, whilst the leading shoot remains apparently sound.
A widely different agency not rarely causes shoots which naturally would have brown out horizontally to grow up vertically. The lateral branches of the Silver Fir (A. pectinata) are often affected by a fungus, Aecidium elatinum, which causes the branch to enlarge into an oval knob formed of hard wood, in one of which we counted 24 rings of growth. According to De Bary*, when the mycelium penetrates a bud beginning to elongate, the shoot developed from it grows vertically upwards. Such upright shoots after-
* See his valuable article in 'Bot. Zeitung,' 1867, p. 257, on these monstrous growths, which are called in German "Hexenbesen," or "witch-brooms." [page 189]
wards produce lateral and horizontal branches; and they then present a curious appearance, as if a young fir-tree had grown out of a ball of clay surrounding the branch. These upright shoots have manifestly changed their nature and become apogeotropic; for if they had not been affected by the Aecidium, they would have grown out horizontally like all the other twigs on the same branches.