BllAUN, ON UNICELLULAR ALG.E. 93 



But to return to the true unicellular AIg(B. The number 

 of genera belonging to this class at present known is small, 

 biit amongst them great diversity exists. The greatest care 

 is requisite in their determination as independent organisms ; 

 nor should this be decided, unless every stage of their evolu- 

 tion from beginning to end is knoAvn. Especially must we be 

 careful not to regard the young state of Alga of a higher 

 order, or depauperate generations [jpaupercidce) , as unicel- 

 lular genera."^ Nor, on the other hand, should less caution 

 be observed not to confound associations of unicellular Algcs 

 with the multicellular. For in several genera of the latter 

 class the closest and most regular associations of distinct 

 individuals, which at first are sometimes freely motile, are 

 met with. These forms are in the highest degree fallacious, 

 presenting as they do a false resemblance to a cellular texture. 

 They are well worthy of more particular attention, and a 

 comparison of them with the families or colonies of the 

 pseudo-unicellular Algce may not be considered superlluous. 

 These compound bodies, however, in Alga, more or less 

 properly speaking unicellular, formed by the association of 

 cells, are so well treated of by Nageli (^Einz. Algen.,^ p, 24), 

 as regards their origin, composition, and diversity of form, 

 that the author has scarcely anything to add beyond certain 

 distinctions, belonging more especially to unicellular AIg(B in 

 the stricter sense of the term. For the associations ofAIgce, 

 less properly so termed, or of the pseudo-unicellular class, are 

 always formed by the vegetative multiplication of cells ; and 

 those of the really unicellular by true propagation : the 

 former, therefore, merely represent individuals divided into 

 more or less independent and loosely coherent parts, and 

 cannot be distinguished by any strict definition from a con- 

 tinuous thallus; whilst the latter are really constituted of 

 several individuals distinct from the first. The associations 

 of pseudo-unicellular A/gte are evolved from a single cell 

 (spore or gonidium) by successive multiplication, the number 

 of cells gradually increasing through a more or less deter- 

 minate series of generations; whilst the associations of the 

 more strictly unicellular Alga, on the contrary, are constituted 

 of several cells [gonidia), distinct ab origine, the number of 

 cells never increasing, owing to the absence of any vegetative 

 division ; the former, in fact, constitute families of cells, pro- 



* Pmtpercida of this kind, that is to say, individuals normally dey)au- 

 perate, and curiously simulating 1-2 cellular parasitic plants, are produced 

 in some species of (Edogonium, as well as in Bulbochate^ from niicrogonidia. 

 ^zrfe Braun, 'Rejuvenescence,' p. 151; Ant. de Bary, in ' Mas. Sekenb.,' 

 1854, pp. 63, 87,' t. iii and iv. 



