SOME REACTIONS OF THE BANANA TO PRESSURE, GRAVITY, AND DARKNESS

Abstract

Great pressure is required to force the emerging leaf of the banana upward through the false-stem. If a section is removed from the side of the false-stem, depriving the sheath of this leaf of its lateral support, it is unable to transmit the pressure exerted by the basal growing zone. The leaf accordingly is not able to emerge, and subsequent leaves break out through the aperture, forming a forked false-stem. When the pressure on the basal growing zone is reduced by cutting off the top of the false-stem, the rate of elongation is immediately increased 5-9 times, but soon falls to its original value. Seedlings placed in a horizontal position erect themselves by a curvature of the base of the false-stem. Plants with false-stems up to 2 m. in height, 20 cm. in basal diameter, and a total weight of aerial portions up to 15 kg., erect themselves from the base when inclined at an angle of 45[degree] or less with the ground. The erection of these plants is accomplished principally by the curvature of the false-stem at its base. The sheaths on the lower side become thicker as a result of the enlargement of their cells without division. There is a decrease of starch on this side. The cortical cells on the lower side of the rhizome elongate parallel to the long axis of the plant and divide by one or more transverse walls. The cell walls become thinner, and the starch grains they contained largely disappear. The whole cortex becomes thicker on the lower side than on the upper. As a result, the false-stem is pushed upward. When a sucker is dug up and replanted in an inclined position, growth of the top is suspended for about a month, while a root system becomes established. Upon renewal of growth, the old false-stem is unable to erect itself. The young leaves, imprisoned within it, burst through the upper side of the old false-stem, at the base, and establish a new false-stem in an upright position. When the plant is placed in darkness, the leaf-sheaths become abnormally long, with the result that false-internodes of about twice the normal length are formed. Sheaths from which the laminae have been removed do not become abnormally long in the dark, although in the open the removal of the lamina does not affect the length of the sheath. When the false-stems are darkened but the laminae remain in the light, the sheaths become only slightly longer than in plants growing in the open. Laminae kept in continuous darkness exert some influence on the basal growing region of their sheaths, about a meter distant, which results in their increased elongation. It seems likely that a hormone is responsible for this effect. There is no reduction in the size of the lamina of plants grown in darkness.