CELLS IN LOWER PLANTS. 99 



tremities. After a short period they fuse, and the resulting 

 mass develops a delicate membrane like the original parent-cell, 

 ;ind two vibratile cilia. 



d. Melosira (Dlatomacece). — Diatoms are members of the Algae, 

 characterised by the possession of silicified cell-walls, which are 

 often beautifully marked. The various markings met with serve 

 in many cases to distinguish the different genera, as also does the 

 enormous variety of shapes which these organisms can assume. 



The cell-wall of a diatom is known as the frustule, and 

 contains enough silica in its composition to enable it to retain 

 its form and markings, even after it has been heated to a white 

 heat. Melosira is here chosen for examination, as this genus 

 shows the structure of the cell very clearly. 



The living cell has the following structure (see Fig. 75) : — 



(a) An external case or eell-Wall, composed in reality of two parts, 

 one of which fits into the other, pill-box fashion. The wall is marked 

 longitudinally by closely-eet parallel lines, which are only apparent under 

 a high power. 



(6) An inner peripheral layer of cytoplasm lining the inner surfaces 

 of the two halves of the frustule; from this layer "bridles" of proto- 

 plasm pass to a central mass, in which is suspended the nUCleuS. 



(c) Two laterally situated masses of a brownish-green colour are to be 

 seen in the cell. These are the Chlorophyll bodies or Chromato- 

 phores. They are semi-fluid in consistency, and internally have sinuous 

 borders. 



Melosira occurs in chains of varying length, there being often 

 ■one hundred or more individuals in a chain. The isolated cell 

 is capable of protruding a portion of its cytoplasm between the 

 two halves of the frustule, and uses this as a pseudopodium for 

 purposes of locomotion. Occasionally, however, diatoms are 

 able to move by causing currents of water to pass through their 

 interior and out again. 



CeU-division in Diatoms takes place lengthwise between the 

 two halves, and the cell-wall of the new individual is enclosed 

 within the ruin of that of the mother-cell, so that repeated 

 •divisions lead to a progressive decrease in the size of individuals. 

 At times, however, large forms known as auxospores arise, and 

 these by their divisions go to produce a smaller series. Auxo- 

 spores arise by the conjugation of two smaller individuals, the 

 resulting cells subsequently decreasing in size on division. 



