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 show some degree of regularity in the arrange- 

 ment 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 to similar 



