THE PROTOPLAST AS HOUSE-BUILDER 



29 



process has it attained even its present growth ? And, still more, how 

 will it develop into the strong-limbed giant which it is destined in future 

 years to become ? 



The answer in part, at least lies in the wonderful property which 

 the protoplasm possesses, not only of building a primary investing wall for 

 itself, but of spreading on the interior of that wall successively new layers 

 of formed material (woody or otherwise in substance, as the case may 

 require) till the cell is all but filled up. This new material, which is found 

 in all Flowering Plants and in very many Cryptogams or Flowerless Plants, 

 is known as secondary de- 

 posit. The process which 

 goes 011 may be likened to 

 the formation of the furred 

 deposit (limestone) on the 

 inside of a kettle. The 

 kettle answers to the cell: 

 the water to the proto- 

 plasm ; the tin side of the 

 kettle to the primary cell- 

 wall ; and the hard lime- 

 stone accretion to the 

 secondary deposit. 



Cellulose itself (C 6 H 10 5 ), 

 though it is the material of 

 which the primary cell-wall 

 is formed, is very seldom 

 found as a secondary de- 

 posit. The date-stone may 

 be cited as -an interesting 

 exception. The thickening 

 which takes place in the 

 interior of the cells of the 

 plum and cherry we do 

 not speak of the stones of 



those fruits and in the pith of certain plants of the Pea family, is 

 a gum ; whilst mucilage, a kind of gum, is found in the cells which 

 form' the seed-coat of linseed, the apple, pear, etc. A very common and 

 important kind of secondary deposit is liguin, which, as might be guessed 

 from the name (Lat. lignum, wood), is found in all woody cells. The stones 

 and shells of many fruits are built up of such cells ; and woody tissue of 

 course abounds in the stems and branches of trees. Lignin, like all 

 secondary deposits, is derived from the protoplasm, which, as the cell- 

 wall increases in thickness, becomes more and more restricted in its 

 movements, until at last it is crowded out, if one may so say, and dies. 



FIG. 49. DIATOMS. 



These are little boxes of pure flint deposited in the interior of micro- 

 scopic plants. Magnified 60 times. 



