INTRODUCTION 



tip or stem tip of a plant shows very beautifully the way in 

 which growth takes place in each of these regions of the plant. 

 Just above the root cap is the region where the cells increase in 

 number by cell division. The cells here are much alike. They 

 are small, thin-walled, polyhedral in shape, with nuclei large in 

 proportion to the size of the cells. The new cells nearest the 

 root cap continue dividing. We call these embryonic cells. Those 

 which are further from the tip 

 no longer divide, but increase 

 considerably in size causing the 

 root as a whole to lengthen. 

 Those cells which have in- 

 creased to near their maximum 

 size are then transformed into 

 their permanent form, assum- 

 ing special shapes, wall modi- 

 fications, and particular func- 

 tions. As you can see from this 

 brief description the growth of 

 the single-celled plant, usually 

 microscopic in size, into the 

 many-celled plant occurs in 

 three stages or steps. There is 

 involved an increase in the 

 number of cells by the process 



of cell division j there is some increase in size of the individual 

 cells formed j and there is a differentiation of these cells into 

 those which make up the leaves, flowers, fruits, seeds, wood, 

 bark, and other parts of the mature plant. In the entire plant 

 no one of the steps is necessarily completed before the next 

 occurs. Although cell division and increase in size predominate 

 in the earlier stages of growth all three may occur, in fact usu- 

 ally do occur, in different regions of one plant at the same time. 

 It is by such a series of events that what was three thousand 

 years ago a single cell, a bit of living jelly less than half the 



Figure 3. Portions of lengthwise sections of 

 a sunflower root. A. The embryonic region 

 and root cap. Note dividing cells in the em- 

 bryonic region. B. In the region of elonga- 

 tion, 2 or 3 millimeters back from the root 

 tip. Cells are increasing in size. C In the 

 region of differentiation. Cells are assuming 

 permanent form. From General Botany by 

 Smith, Overton, Gilbert, Denniston, Bryan, 

 and Allen. By permission of the Macmillan 

 Company. 



