46 PLANT LIFE ON THE FABM. 



the molecules of cell-membrane are separated by the pres- 

 sure caused by the turgid protoplasm, and into the inter- 

 stices so formed, new molecules of membrane formed by 

 the protoplasm are, as it were, squeezed. The process is 

 as if a number of grains of sand were laid upon a table, 

 each grain just touching its neighbor, and then a new 

 grain were forced in between two others, only in this case 

 the new grain is formed in the cell itself. The requisite 

 pressure is afforded, in the case of the cell, by the grow- 

 ing protoplasm within, and by the influx of fluid into 

 the cell by osmosis, producing a condition of turgescence. 

 The growth of the protoplasm itself takes place precisely 

 in the same way as that of the cell membrane viz., by 

 the formation of new particles, which are squeezed in by 

 intercalary growth between the older ones. New matter 

 is also deposited on the outer surface of the protoplasm 

 or inner surface of the cell wall. 



Lastly, growth is effected, not merely by extension of 

 old cells, or by incorporation of new materials with old, 

 but by actual increase in the number of cells. This 

 increase in number has been brought about by the sub- 

 division of the protoplasm into two or more segments, 

 each of which becomes invested by cell-membrane. 



For full details as to the various ways in which division 

 of the protoplasm and the formation of new cells take 

 place, reference must be made to text books. What has 

 been here said is sufficient to indicate the general nature 

 of growth in the organs with which we are here most 

 concerned the root, the stem, and the leaf. 



Growing Points. As has been stated, all the parts of 

 plants are at first wholly cellular and structurally indis- 

 tinguishable ; but, as growth goes on, not only their 

 outer form alters, but the form and arrangement of their 

 constituent cells also, so that various tissues fibrous, 

 woody, vascular, or epidermal are formed ; and thus it 



