INTRODUCTION. 5 



in size, acquire colour, and gradually put on the appearance 

 of the old portions. Of course, as they increase the original 

 segments are pushed farther asunder, and at length are dis- 

 connected, each taking with it a new segment to supply the 

 place of that from which it has separated. 



It is curious to trace the progressive development of the 

 new portions. At first they are devoid-of colour, and have 

 much the appearance of condensed gelatine, but as they in- 

 crease in size the internal fluid acquires a green tint, which 

 is at first very faint, but soon becomes darker ; at length it 

 assumes a granular state. At the same time the new seg- 

 ments increase in size and obtain their normal figure ; the 

 covering in some species shows the presence of puncta or 

 granules ; and lastly, in Xanthidium and Staurastrum the 

 spines and processes make their appearance, beginning as 

 mere tubercles, and then lengthening until they attain their 

 perfect form and size ; but complete separation frequently 

 occurs before the whole process is completed. This singular 

 process is repeated again and again, so that the older seg- 

 ments are united successively, as it were, with many gene- 

 rations. In Sphserozosma the same changes take place, and 

 are just as evident, but the cells continue linked together, 

 and a filament is formed, which elongates more and more 

 rapidly as the joints increase in number. This continued 

 multiplication by division has its limits ; the segments gra- 

 dually enlarge whilst they divide, and at length the plant 

 ceases to grow ; the division of the cells is no longer repeated ; 

 the internal matter changes its appearance, increases in 

 density, and contains starch-granules which soon become 

 numerous ; the reproductive granules are perfected, and the 

 individual perishes. In a filament the two oldest segments 

 are found at its opposite extremities ; for so long as the 

 joints divide they are necessarily separated further and 

 further from each other. Whilst this process is in progress 

 the filament in Sphserozosma consists of segments of all sizes ; 

 but after it has reached maturity there is little inequality 

 between them, except in some of the last-formed segments, 



