THE EFFECT OF PHYSICAL AGENTS (ESP. GRAVITY) ON THE TISSUE DEVELOPMENT OF THE MAIN AXIS OF SOME PLANTS.

Abstract

1. It has been observed how the histological changes are caused on the tissue structure of the hypocotyl or stem by turning the direction of gravity acting upon it. 2. a) When the stem of a tomato plant is forced to maintain its horizontal position in the direction of gravity acting upon it, its growth in thickness in the treated part is more accelerated at the lower tissues than at the upper ; thus it presents an appearance of hypo-trophyical growth. b) This phenomenon is due to the dissimilarity in the growth of each individual cell, and not to the numerical increase or decrease of cell layers. c) The histological dissimilarity between the upper and the lower side is most noticeable in the cortex ; thus the cell walls of the upper collenchym grow strikingly thick, while the corresponding cell walls in the lower side have uniform thickness ; therefore, in the lower part such a collenchym as seen in the upper does not develop. Both the phloem and xylem cells in the lower side are more elongated in the radius direction of the stem, their walls become more thinner, and larger vessels are formed in greater number. d) The guard cells as well as the epidermal cells become larger in the lower side. e) Each tissue which forms the flank of the horizontal stem shows, with a gradual transition, an intermediate state between the upper and the lower side. 3. a) The hypocotyl or stem of Indian mallow plant, when held up horizontally, grows epitrophically in contrast with the stem of a tomato plant similarly treated. This growth in thickness is mainly due to the increase of the number and size of cells in the cortex and vascular bundles. b) In the upper side, cells which constitute the outer 3∼4 layers of the cortex come to assume flat and irregular shapes as a result of a higher pressure caused by the growth of the vascular bundles as well as the cell division occurring in the deeper part of the cortex, while in the lower side they maintain the spherical shape because of a lower pressure from the inner part. c) The formation of the phloem fibres is accelerated in the upper side, and retarded in the lower side ; thus it is recognizable that there arises a great quantitative dissimilarity in the fibre formation between these sides. And also in the upper side the fibrous cell walls grow thicker, lignification becoming higher than the corresponding cell walls in the lower side. The cells of each bundle of fibre are so arranged as to be consistent with radius direction of the axis in the upper side, and in the lower side they are arranged along the parallel direction around the axis, so that an elongated shape in the upper side, while in the lower it is found in a small lump. Moreover, the phloem fibres in the upper side, are pushed aside, either to the left or the right, from the zenith of the horizontal hypocotyl to its lower flank, as the dissimilarity is so great in the growth of the vascular bundle between the upper and the lower. Thus the phloem fibres are sparsely distributed in the upper, while densely in the lower. d) The dissimilarity of the growth of the xylem part between the upper and the lower side is also very great ; the thickness of the upper xylem 3 times as much as that of the lower xylem, and the number of cell layers are in the ratio 2.5∼3 : 1 : But the lower side surpasses far the degree of their lignification. 4. The dissimilarity of tissue development occurring in both the upper and lower side of one and the same hypocotyl or stem is due mainly to the change of the acting direction of gravity, so the effect of sun's heat and light can be safely disregarded. 5. Erom the results of experiments, the following two points may be assumed ; the bringing about of an unequal distribution of plant growth hormone by turning the direction of gravity acting upon the main axis of the plant and the specificity of reaction to this substance, characteristic of separate plant species or variety.