The foregoing cases with respect to seedling plants have been fully described, not only because the transmission of any effect from light is a new physiological fact, but because we think it tends to modify somewhat the current views on heliotropic movements. Until [page 485] lately such movements were believed to result simply from increased growth on the shaded side. At present it is commonly admitted* that diminished light increases the turgescence of the cells, or the extensibility of the cell-walls, or of both together, on the shaded side, and that this is followed by increased growth. But Pfeffer has shown that a difference in the turgescence on the two sides of a pulvinus,--that is, an aggregate of small cells which have ceased to grow at an early age,--is excited by a difference in the amount of light received by the two sides; and that movement is thus caused without being followed by increased growth on the more turgescent side.** All observers apparently believe that light acts directly on the part which bends, but we have seen with the above described seedlings that this is not the case. Their lower halves were brightly illuminated for hours, and yet did not bend in the least towards the light, though this is the part which under ordinary circumstances bends the most. It is a still more striking fact, that the faint illumination of a narrow stripe on one side of the upper part of the cotyledons of Phalaris determined the direction of the curvature of the lower part; so that this latter part did not bend towards the bright light by which it had been fully illuminated,
* Emil Godlewski has given ('Bot. Zeitung,' 1879, Nos. 6-9) an excellent account (p. 120) of the present state of the question. See also Vines in 'Arbeiten des Bot. Inst. in Würzburg,' 1878, B. ii. pp. 114-147. Hugo de Vries has recently published a still more important article on this subject: 'Bot Zeitung,' Dec. 19th and 26th, 1879.
** 'Die Periodischen Bewegungen der Blattorgane,' 1875, pp. 7, 63, 123, etc. Frank has also insisted ('Die Naturliche wägerechte Richtung von Pflanzentheilen,' 1870, p. 53) on the important part which the pulvini of the leaflets of compound leaves play in placing the leaflets in a proper position with respect to the light. This holds good, especially with the leaves of climbing plants, which are carried into all sorts of positions, ill-adapted for the action of the light. [page 486]
but obliquely towards one side where only a little light entered. These results seem to imply the presence of some matter in the upper part which is acted on by light, and which transmits its effects to the lower part. It has been shown that this transmission is independent of the bending of the upper sensitive part. We have an analogous case of transmission in Drosera, for when a gland is irritated, the basal and not the upper or intermediate part of the tentacle bends. The flexible and sensitive filament of Dionaea likewise transmits a stimulus, without itself bending; as does the stem of Mimosa.
Light exerts a powerful influence on most vegetable tissues, and there can be no doubt that it generally tends to check their growth. But when the two sides of a plant are illuminated in a slightly different degree, it does not necessarily follow that the bending towards the illuminated side is caused by changes in the tissues of the same nature as those which lead to increased growth in darkness. We know at least that a part may bend from the light, and yet its growth may not be favoured by light. This is the case with the radicles of Sinapis alba, which are plainly apheliotropic; nevertheless, they grow quicker in darkness than in light.* So it is with many aërial roots, according to Wiesner;** but there are other opposed cases. It appears, therefore, that light does not determine the growth of apheliotropic parts in any uniform manner.
We should bear in mind that the power of bending to the light is highly beneficial to most plants. There
* Francis Darwin, 'Über das Wachsthum negativ heliotropischer Wurzeln': 'Arbeiten des Bot.