PLANT LIFE. 



consists of two halves ; the older half is slightly larger than 

 the younger one, and fits on to the latter like the lid of a 

 box. This siliceous wall is covered externally by a thin 

 •film of exuded protoplasm, and it 

 is now generally considered that 

 the spontaneous movements ex- 

 hibited by many species is due to 

 the contractility of this external 

 protoplasm. In the majority if 

 not all species the two portions or 

 valves of the cell-wall are furnished 

 with numerous rows of exceed- 

 ingly minute perforations, which 

 unless very highly magnified pre- 

 sent the appearance of fine ridges. 

 Three modes of reproduction 

 are known, (i) By division. The" 

 first indication of this process is 

 the division of the nucleus ; the 

 two valves separate and the proto- 

 plasm becomes resolved into two 

 daughter-cells, each of which be- 

 comes enclosed in a cell-wall 

 „. , , , „. , . formed of two valves, the smaller 



Fig. 10. — \y.t^ip). Pinnularia . 



viridis, a diatom furnished of the two being formed last. As 



with a siliceous cell-wall. ^, , r^i j i-i. n ., 



A, side view; B, edge view, the valves of the daughter-cells are 



The central shaded portion formed within the old valves of 



IS the protoplasm enclosing 



a nucleus. (From Stras- the mother-cell, it necessarily fol- 

 ""^^^"^ lows that each generation formed 



by this method becomes smaller than the preceding one. 

 The original size is restored by a second form of repro- 

 duction, where what is termed an aiixospore is formed by 



