10 



plastids. Both these are, however, only modifications of 

 the same organ. 



The Rhodoplastids are best developed in the assimilating 

 cells (fig. 14, 15). They are seen here to be of a dark red 

 colour. The red colour is made up of a mixture of chloro- 

 phyll and phycoerythrin, the latter completely obscuring 

 the former. The latter may also be extracted by sub- 

 mersion in fresh water for some time, preferably in waim 

 water. The plant will remain green, the chlorophyll 

 being insoluble in water. 



The outermost cell of the assimilating filament has a 

 very small rhodoplastid. The latter is represented by a 

 very much reduced flat structure, which fits into the outer 

 end of the oval shaped cell. The remaining part of the 

 cell appears colourless. The other assimilating cells 

 possess very well developed dark red rhodoplastids. They 

 form here cylindrical plates, which line two or three or 

 even all the radial walls, and sometimes even the outer 

 tangential wall. They do not form a closed cylinder, for 

 they are open along one side. Each cell here contains 

 only one rhodoplastid. 



The Ehodoplastids are well developed in these assimi- 

 lating cells, but as you pass on to the collecting cells, 

 thev srraduallv change. The red colour becomes fainter, 

 they get drawn out and become very finely divided. When 

 we get nearer to the conducting cells, the rhodoplastids 

 have become almost invisible. A^ery finely divided 

 narrow strands are seen of a very faint pink colour. These 

 are the plastids. The fine strands are interrupted here 

 and there by rather larger and more deeply stained masses. 

 Finally in the most central of the conducting cells the 

 finely divided rhodoplastids have disappeared, their place 

 being taken by small roundish leucoplastids. These are 

 almost colourless, but often show a very faint greenish 



