The Journal of Indian Botany/Volume 1/March 1920/Note on the Geotropic Curvature of the Inflorescene in Eichhornia Speciosa KUNTH (Water Hyacinth)
NOTE ON THE GEOTROPIC CURVATURE OF THE INFLORESCENE IN EICHHORNIA SPECIOSA KUNTH (WATER HYACINTH.)
BY
P. S. JlVANNA RAO, M.A.
Agricultural College and Research Institute, Coimbatore.
There are many striking peculiarities about the Water Hyacinth of which the bending of the inflorescence after flowering appears to be very noteworthy. The bending is seen in several stages but observation at definite intervals extending for a few days brings out certain interesting features which are briefly noticed below.
The inflorescence is an elongated erect spike with about 10-15 lilac or mauve coloured flowers arranged at the top of a long shoot. Flowers are medianly zygomorphic conforming to the formula P 3 + 3. A 3 + 3, G (3). There are two sheathing bracteal leaves, inserted one below the other, the lower having a distinct lamina.
The flowers in each inflorescence open simultaneously at about 8 o'clock in the morning, though occasionally a few may lag behind and open on the next day at the same hour.
With a view to ascertain the exact time of bending of the inflorescence I employed the Ganong's auxograph and placing the plant in position connected the tip of the inflorescence with a thread passing round the larger wheel. By tracing the curve on the cylinder which revolves once in an hour I found on the following morning that the curvature actually commences at about 10 in the night. Plants were also kept separately under observation and the interesting fact was noticed viz., that the closing of the flowers and the bending of the inflorescence occur simultaneously at about 10 P.M.
That the curvature is due to geotropism was proved in the following manner. A plant with flowers just open was fitted in a pot which was loosely packed with wet sponge so as to keep the roots moist. The pot was then fixed to the disc of a klinostat and rotated with the plant held horizontally. The roots were further kept moist by being watered at intervals. Though the flowers had closed as usual there was no curvature of the inflorescence even after three days. Hydrotropism does not play any part for the curvature took place when a plant was adjusted with the inflorescence horizontal and completely immersed in water. This shows that the curvature is the result of geotropism only. The several stages in the process of bending are shown in the figure below. The actual bending is due to one sided growth at the portion immediately below the insertion of the two sheathing leaves. Examination of this part of the shoot before and after the curvature by marking it with india ink made this clear and it was further noticed that greater growth was as a rule opposite to the bladed sheath so that this was always lower in the curved inflorescence. A rapid bending is seen when the plant is in its best state of growth and the curvature is at two places, one at the base of the shoot and the other just beneath the bracteal leaves referred to above. The bends also appear as loops owing to the basal portion of the shoot being slow of response. The loops were observed in those cases where there was a delay in the development of the topmost flowers. The mutilation of such flowers brought about a similar result as also the complete removal of flowers just opened or of the inflorescence with unopened flowers a little above the insertion of the bracts. The loops were generally characteristic of plants in a poor state of growth. It is interesting to note that the > shaped curvature is reached on the sixth or seventh day of the opening of flowers.
A close study of the phenomenon as explained above shows that we are really dealing with two kinds of geotropic curvatures (1) the positive geotropism of the inflorescence resulting in its complete reversal and caused by a curvature beneath the insertion of the bracts, and (2) a diageotropism of the lower portion of the flowering shoot which corresponds to the internode of the sympodial vegetative axis and behaves as such
