The Unilateral Effect of the Gravity upon the Growth in Thickness and Tissue Structure of Some Chrysanthemum Plants

I) The axes of sunflower plant, Helianthus annuns, and cosmos plant, Cosmos bipinnatus, when held up horizontally, grow more in thickness than those of vertical ones, and judging from their shapes of cross sections as well as their histological structures, it may be regarded that both of them should be classified as a type of hypotrophieal growth in general. a) The cross section of the horizontal bypocotyl of sunflower plant presents nearly a round shape as the result of counterbalance in which the cortex at the lower side becomes thicker on one hand, and the cells in the upper part of pith elongate upward more than the opposite side, on the other hand, respectively. b) The cross section of the horizontal stem of sunflower plant show an elliptical shape in which the longitudinal axis is longer than the transversal one. This is due to the fact that the extent of upward elongation of the cells is very conspicuous in the upper part of pith, while the extent of growth in thickness of cortex is comparatively little in the lower side. c) In the horizontal hypocotyl of cosmos plant of which one side was brought to the upper, that side curves and projects upward as it is forming a dome and when one corner is brought to the upper, its cross section presents a diamond shape in which the upper part is more narrow and the lower pai't wider. This is due to the fact that in both the hypocotyl and vtem the pitli cells elongate remarkably upward, though the cortex grows more in thickness at the lower side than at the upper. d) The change in a shape of the cross section of the stem of cosmos plant which is brought about when the stem is maintained horizontally, has a same tendency to that of a hypocotyl treated alike, but it occurs occasionally the case in which it does not much differ from the shape of the cross section of the stem kept vertically, apart from the difference in the shape of cortical and pith cells between the upper and the lower side. This will be attributable to the fact that the cortex is thinn and the effect of an upward elongation of the pith cells in the upper part are mechanically suppressed on account of a dense arrangement of smaller vascular bundles all around the periphery in the stem.II) The horizontal axes of both thesunflower and the cosmos plants have a similar tendency on the tissue structures. The noticeable points shown on the cross sections are as follows. a) While the cortex in the upper side is composed of smaller cells with thicker walls, that in the lower side is composed larger cells with thinner walls, but does not lose the characteristic features as eollencliyni. Thus, the thickness of the cortex in the lower side becomes thicker than that in the upper side, though the number of cell layers does not differ between both sides. b) The vascular bundles are densely formed in the upper side, sparsely in the lower side. The xylem formation and its lignification are more accelerated in the upper side than in the lower. The xylem cells in the upper side are smaller, their walls thicker and arrange more densely than those in the lower side. The cells which are differentiated from cambium in the upper side has a like appearance with those shown in the control plant, but these in the lower side are roundish and show the intercellular spaces. c) As the cells in the upper part of the pith elongate upward and those in the lower part maintain an original shape, the pith grows upward.III) The hypotrophy shown by the horizontal axes of sunflower and cosmos plants will be due to the fact that the growth hormones are much accumulated in the lower side of the plant axis followed by a change in the direction of gravity to it. Since the growth hormones come to be effectual in connection with some enzymes, the fact that the mode of growth in thickness which is shown by the horizontal axis is different at every tissue, as seen in the present experiments, may be depend upon the dissimilarity of quantity and distribution of such enzymes in every tissue