1 8 BOTANY OF THE LIVING PLANT 



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 protoplasmic body, or 



FIG. 9 



Young thin-walled cells from the growing point of Tradescantia, each with a 

 relatively large nucleus, containing a highly refractive nucleolus. Many plasticl- ;i n 

 present in the cytoplasm. After Schimper. ( x 800.) 



protoplast, is applied to all the contents enclosed 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 the 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 

 a division of a pre-existent mother-cell into two, usually equal 

 daughter-cells. 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 show some degree of regularity 

 in the arrangement of the cells that are formed. In some cases that 

 regularity is very striking. The scheme of construction in the case 

 of the apex of Hippuris would be like that shown in Fig. 10, and it is 

 found that in plants at large the young tissues are arranged according 



