410 THE MICROSCOPE. 



appears like two, the segments being always distinct, even from the 

 commencement. As the connecting portion is so small, and neces- 

 sarily produces the new segments, which cannot arise from a broader 

 base than its opening, these are at first very minute ; though they 

 rapidly increase in size. The segments are separated by the elongation 

 of the connecting tube, which is converted into two roundish hyaline 

 lobules. These lobules increase in size, acquire colour, and gradually 

 put on the appearance of the old portions. Of course, as they increase, 

 the original segments are pushed further asunder, and at length are 

 disconnected, 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 por- 

 tions. At first they are devoid of colour, and have much the appear- 

 ance of condensed gelatine; but as they increase in size, the internal 

 fluid acquires a green tint, which is at first very faint, but soon be- 

 comes darker ; at length it assumes a granular state. At the same 

 time the new segments 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, and 

 armed with setaa; but complete separation frequently occurs before 

 the whole process is completed. This singular process is repeated 

 again and again, so that the older segments are united successively, as 

 it were, with many generations. When the cells approach maturity, 

 molecular movements may be at times noticed in their contents, pre- 

 cisely similar to what has been described by Agardh and others as 

 occurring in the Confervse. This movement has been aptly termed a 

 swarming. All the Desmidacese are gelatinous. In some the mucus 

 is condensed into a distinct and well-defined hyaline sheath or cover- 

 ing, as in Didymoprium Grevittii and Staurastrum tumidum ; in others 

 it is more attenuated, and the fact that it forms a covering is discerned 

 only by its preventing the contact of the coloured cells. In general 

 its quantity is merely sufficient to hold the fronds together in a kind 

 of filmy cloud, which is dispersed by the slightest touch. When they 

 are left exposed by the evaporation of the water, this mucus becomes 

 denser, and is apparently secreted in larger quantities, to protect them 

 from the effects of drought. Meyen states he has " distinctly seen that 

 the large and small granules contain starch, and were sometimes even 

 entirely composed of it ;" and that " in the month of May he had ob- 

 served many specimens of Closterium in which the whole interior sub- 



