FLINT CELL-WALLS. 



vegetable kingdom ; for example, in diatoms, a group of 

 microscopic algae, the cell-wall is rendered rigid with 

 silex or flint, and it maybe partly 

 owing to this selection that dia- 

 toms at the present day form the 

 terminal members of the group 

 that adopted this speciality, the 

 disadvantages connected with a 

 perfectly rigid wall of flint as com- 

 pared with the yielding wall of 

 cellulose, other things being equal, 

 is sufficiently obvious. It would 

 appear that the adoption of flint 

 for the cell-wall must have been 

 in connection with protection, 

 and it has recently been sug- 

 gested that in the diatoms it is 

 an acquired character, and for 

 the purpose of protecting these 

 organisms in their passage through 

 the alimentary canal of such aqua- 

 tic animals as frogs, which swal- 

 low large quantities along with 

 their food. A considerable quan- 

 tity of silica is present along with 

 cellulose in the outside cells of 

 the stem and branches of the 

 various species of "horsetail" 

 {Equisetum), also on the surface 

 ofbamboo and cane(palm) stems; the cutting action of blades 

 of grass when drawn between the fingers is also due to 

 particles of flint in the cell-wall. 



Fig. 4. — (x 540). Pinnularia 

 viridis, a diatom furnished 

 with a siliceous cell-wall. A , 

 side view ; B, edge view. 

 The central shaded portion 

 is the protoplasm enclosing 

 a nucleus. (From Stras- 

 burger.) 



