The Fine Structure of Quiescent and Growing Carrot Cells: Its Relation to Growth Induction

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Abstract
This paper records a survey of the fine structure of carrot cells. Cells have been examined in the state in which they occur in the dormant root, and under the influence of stimuli which cause them to grow and divide rapidly. Although the carrot cells, which are often highly vacuolate with very thin parietal cytoplasm, presented special technical difficulties, these were overcome. The information which has been obtained has special significance because it can be related to cells that undergo a recrudescence of growth brought about by known agents and conditions. The induction of growth in otherwise resting tissue is manifest at all levels from protoplasmic streaming, the position and division of nuclei, the abundance of cytoplasmic strands, all seen under the light microscope, as well as effects which are only visible with the electron microscope on the ground substance of cytoplasm and upon all the organelles that have been investigated. By deliberate choice a variety of techniques have been employed; some of these emphasize the membrane components of the cytoplasm and its organelles, others stress their granular non-membranous content. In this way sharply contrasting pictures of plastids and mitochondria are obtained. Some methods show the cell wall with its cellulosic fibrils conventionally revealed; others demonstrate it in the form of non-cellulosic droplets or granules obviously mounted upon very fine supporting threads. By all these means the quiescent and actively growing cells are contrasted with reference to each of their prominent features. Ribosomes, both free and attached to membranes, are conspicuously abundant in the activated cells and this correlates with the known relation of the nucleic acid metabolism to their growth and protein synthesis. The endoplasmic reticulum, mitochondria, plastids, Golgi bodies, are all conspicuous in the activated cells which can divide and there are characteristic differences in their form and structure which can be seen during growth induction. The new cell wall at cytokinesis (middle lamella) has been seen as well as the formation of protoplasmic connexions, which unite the derivatives of one cell into an organized structure. The later events that occure in the formation of an air space, which permit valuable observations on cell walls to be made, have also been described. Thus the electron microscope is now a valuable tool to supplement studies on the biochemical effects of growth-regulating substances and in the understanding of growth induction; aseptically cultured tissue explants and free cells, used under conditions in which growth and morphogenesis may occur, constitute equally valuable material for the investigation of the fine structure of cells and its relation to development.

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