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PLANT GROWTH 



Strands running to each wall appear. It is these which indicate the plane of divi- 

 sion, for the one which lies in the position of the future wall becomes much thicker; 

 this heavy persistent cytoplasmic strand is called the phragmosome (Sinnott and 

 Bloch, 1941)- The metaphase plate clearly lies in the plane of the phragmosome 

 (Fig. 4B). 



Fig. 4. Location of new cell-walls in meristematic tissue. A. Walls or phragmosomes 

 opposite one another; transverse section through wounded petiole o{ Bryophylhim. B. Walls 

 or phragmosomes avoiding the opposite position; longitudinal section through normal shoot 

 tip of Polvgomim. In the forked phragmosome (left) only one branch will persist. C. Walls 

 and phragmosomes parallel to wounded surface (indicated by line at left) ; portion of wound 

 meristem of Tradescantia. (All from Sinnott and Bloch, 1 941). Camera lucida drawings. 



The cell-wall initial, or phragmoplast, is apparently formed at first not from 

 the spindle fibers themselves, but from droplets arising between the spindle fibers; 

 however, parts of the spindle fibers become incorporated in it. As soon as it is form- 

 ed, the phragmoplast expands laterally by adding new short fibrils at either end 

 till it reaches the lateral walls. In cells below the apical meristem the plane of the 

 new wall is, of course, in most cases transverse; in wounded tissue, on the other 

 hand it lies parallel to the plane of the wound (Fig. 4Aand C). These orientations 

 have been ascribed to the difixision gradient of auxin or of wound hormone, but 

 with little factual support. The positions of the new walls are commonly opposite 

 one another in wounded tissue, but alternate in normal meristems (Fig. 4). 



In the cambium the plane of division is lengthwise in a long cell. The remarkable 

 process of cell division in cambium cells of the Coniferae, such as white pine, has 

 been fully described by Bailey (191 9, 1920). These cells, although uninucleate, 

 may be several hundred times as long as they are wide. The mitotic figure is at 

 angle of 20-40° to the long axis of the cell. The spindle fibers expand laterally, 

 after karyokinesis, by adding small fibrils peripherally; the expanding cell plate is 

 thus oblique at first but it soon curves to become longitudinal. As it loses the fibers 

 in the central part, to form the cell plate, it adds a mass of fibrils at its end (Fig. 5) 

 and these move away from one another at equal rates towards the ends of the cell, 

 which may be up to 4 mm away. In this way the divided nucleus is always at the 

 center of the expanding cell plate. The process takes so long that the nuclei are 



