POLYSIPHONIA. 39 



as such, since the pit- membrane is covered on either side by 

 thick plates of a highly -refractive substance ("stoppers" of 

 Wright), which are themselves, however, connected by fine 

 strands passing through the pit-membrane. 



Valuable information on various points is to be gained by 

 treating fresh specimens of thalli, which do not bear reproductive 

 organs, with slightly dilute sulphuric acid for about an hour : 

 material thus prepared is to be mounted in water or weak 

 glycerine, and by a slight pressure with a needle on the cover- 

 slip the protoplasmic bodies of the cells may be disengaged from 

 the greatly swollen cell- walls : they will often be seen to hang 

 together in groups, and the finer protoplasmic strands (with a 

 more or less obvious sieve arrangement) will then be seen ex- 

 tending between them : a subsequent staining with Hoffmann's 

 blue will make this more apparent. Special attenton should be 

 given to the subdivision of the successive segments cut off from 

 the apical cell, since the pericentral cells are derived from them 

 in a peculiar manner. It may be seen that they are formed, not 

 simultaneously, but successively. The first is cut off in form 

 of a wedge, and the rest of the pericentral cells appear alternately 

 on either side of the first, until the whole periphery of the 

 segment is occupied by a series of pericentral cells surrounding 

 the central one from which they are derived. 



A careful observation of young apices externally, under a high 

 power, will be found to bear out this observation. 



IV. Longitudinal sections should also be cut so as to 

 complete the study of the tissues: in median longi- 

 tudinal sections the series of central cells will be obvious 

 under a low power, the dark protoplasmic body being 

 surrounded by a swollen and beautifully stratified cell- 

 wall. Externally will be seen the pericentral cells of 

 oblong form, their free walls being covered by the 

 continuous cuticle. Pay special attention, under a high 

 power, to the protoplasmic strands, which connect the 

 cells of the central series one with another, and also 

 with the pericentral cells. The former are thick strands, 



