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CHAPTER IV. 

 CELL GROWTH, SHAPE, FORMATION, AND TISSUES. 



Cells grow by the process of intussusception. This is a 

 term which requires some amount of explanation. We must 

 look upon the cell wall as consisting, not of a continuous 

 homogeneous layer of cellulose, but of an immense number 

 of isolated, minute, solid particles or molecules, which are 

 comparatively speaking unalterable. 



Between these it is supposed that water percolates so that 

 each molecule is surrounded by a layer of liquid. The neigh- 

 bouring molecules may vary in size, so that, if the aqueous 

 envelope remains the same, larger molecules will form a denser, 

 smaller a less dense, substance. 



This hypothesis (for it is but an hypothesis, as the mole- 

 cules are too minute to be perceived even by the strongest 

 lens) was first suggested by Nageli, and it is supposed that 

 these molecules are held in their places by three forces, viz. : 

 I. The cohesion of the particles of which each individual 

 molecule is made up. 2. The mutual attraction which exists 

 between the adjacent molecules, and which gives them a ten- 

 dency to approach one another. 3. The attraction of their 

 surfaces for the watery envelope which counteracts this ten- 

 dency. 



When fresh food material is brought to the cell, the new 

 molecules are intercalated between those already existing there, 

 and thus they cause an increase in the size of the cell wall. 

 Such a process of increase by means of intussusception is very 

 characteristic of the growth of organic beings, and is totally 

 different from the manner in which minerals increase in size. 

 If a crystal, say of alum, be suspended in a vessel containing a 



