324 Mr. J. Ralfs on the Nostocliinesp. 



often rendered useless by the separation of the filament into 

 single cells. The destruction of the filament is attended by the 

 escape of the colourin^^ matter, which stains the water or what- 

 ever is in contact with the mass, and is usually the first sign of 

 that destruction. 



I find the best method of preserving specimens is to dry them 

 as quickly as possible on talc or glass. Specimens preserved on 

 paper can rarely be removed without injury. In examining spe- 

 cimens that have been dried, it is necessary to bear in mind, that 

 although, when revived by adding a little water, they present cha- 

 racters apparently but little altered from their recent ones, yet 

 their joints are then more distinct and orbicular from contraction 

 at their junction ; hence a cell quadrate in the recent plant will 

 be orbicular in the revived one. I have elsewhere mentioned 

 that from a similar cause the dried frond in Closterium appear* 

 more attenuated at the extremities than is natural, and I fear 

 that from inattention to this fact descriptions taken from dried 

 specimens are sometimes faulty. 



Until the publication of Professor Kiitzing^s * Phycologia Ge- 

 neralis,^ the described sjiecies belonging to this group were few 

 in number, and usually retained in a single genus either as Ana- 

 haina, Bory, or Sjjharozyga, Ag. Professor Kiitzing has now 

 determined upwards of thirty species, which he has distributed 

 in four genera*. 



Attempts to ascertain the earlier synonyms in tribes which 

 require the aid of the microscope to detect the generic and spe- 

 cific difi*erences are necessarily attended with much difficulty. 

 Not only are our present instruments far superior to those used 

 a few years ago, but when natural history began to take its 

 proper rank in science, the higher tribes sufficiently taxed the 

 time and skill of collectors and writers ; it is therefore not sur- 

 prising that the more minute Cryptogamia should have been 

 comparatively neglected. The descriptions were chiefly taken 

 from characters obvious to the naked eye, and besides were often 

 so brief and at the same time so vague, that they were equally 

 api)licable to members of very different genera ; hence authors, 

 unable to determine with certainty the species of their prede- 

 cessors, were frequently compelled either to depend on chance in 



* I take this oj)poitunity of directing attention to his ' Tabulas Phycolo- 

 gicie/ now publishing in numbers in a cheap form, and containing magnilied 

 figures of every species known to him. 'Jo tliose who wish to identify our 

 JJiitish freshwater Algse it is indispensable. Of British species of Oic/V/a- 

 toihi we have no figures of the slightest value, for unfortunately Mr. Hassall^ 

 ninny of whose figures in other genera are very useful, has, in every figure 

 which he has given of that genus, omitted to give the ends of the filaments, 

 though they are often essential to the determination of the species. 



