. Botany of the living plant. Botany; Plants. 18 BOTANY OF THE LIVING PLANT In a central position in each of them is a more highly refractive, spherical body : this is the nucleus. Embedded in the cytoplasm, and often difficult to observe, are other minute roundish bodies, which are colourless : they are the plastids. The collective term proto- plasmic body, or protoplast, is applied to all the contents enclosed. Fig. 9. Young thin-walled cells from the growing point of Tradescantia, each with a relatively large nucleus, containing a highly refractive nucleolus. Many plastids are present in th
. Botany of the living plant. Botany; Plants. 18 BOTANY OF THE LIVING PLANT In a central position in each of them is a more highly refractive, spherical body : this is the nucleus. Embedded in the cytoplasm, and often difficult to observe, are other minute roundish bodies, which are colourless : they are the plastids. The collective term proto- plasmic body, or protoplast, is applied to all the contents enclosed. Fig. 9. Young thin-walled cells from the growing point of Tradescantia, each with a relatively large nucleus, containing a highly refractive nucleolus. Many plastids are present in the cytoplasm. After Schimper. ( x 800.) The minute size of the cells may be realised by measurement of their diameter as seen in Fig. 9, and division of the results by the magni- fication as stated : the result is a diam. of about -02 mm. within the cell-wall. In older tissues the cell-walls are often so conspicuous that the units of construction were called " cells " by the earlier observers, from their comparison with the partitioned honeycomb. That name is still retained for them. But it is now fully recognised that it is the protoplast and not the cell-wall that is the essential part, for it is in it that the active vitality is centred. Increase of Cells by Division. As the tissues increase with the general growth of the apical region, the number of cells composing it increases by cell-division. An examination of the tissues themselves will show how this is carried out. Very frequently cells may be found in the apical cone grouped in pairs, and separated by a very thin wall. These plainly indicate that a division of a pre-existent mother-cell has recently taken place, so as to form two usually equal daughter-cells from one parent cell. The new cell-wall thus formed is inserted at right angles upon the older walls. If the cells always divide into nearly equal halves, and if the new walls are fixed at right angles upon the older walls, the result must necessarily sho
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