INTRODUCTION TO CRYPTOGAMIC BOTANY. 121 



again divides. But this is by no means the case with all, for 

 the two original halves do not always separate, but remain united 

 with their progeny for many generations, thus forming a fili- 

 form body (Fig 7, a), in which the two primary halves are at 

 either extremity, and the youngest in the middle of the thread : 

 a mode of increase which we shall meet with again in the fol- 

 lowing tribe. Nor is the connection always confined to deve- 

 lopment in a straight line. In those species which divide 

 obliquely, an orbicular frond is sometimes formed, as in Pedi- 

 astrum, though seldom one of any remarkable size. The 

 fronds, if they may be so called, or cells, differ in almost every 

 conceivable way, from a right-lined parallelogi-am or curved bow, 

 to a deeply pinnate or strongly serrated thread. They are, for 

 the most part, strongly constricted, so as to appear like two dis- 

 tinct cells ; but this is by no means universal, for in ClosteriuTn 

 there is no greater constriction than in many species of Con- 

 ferva. They are either smooth, finely granulated, verrucose, or 

 beset with forked spines, insomuch that nothing can well be 

 more curious than the varied outlines which they present 

 under the microscope. Besides the spines, in the genus Xan- 

 thidium, there is, in the centre of each half-cell, on either 

 side, a curious tubercle, the edge of which is sometimes granu- 

 lated, but whether it has any especial function or not, is un- 

 known. Mr. Kalfs doubts whether the increase of the fronds, 

 by separation, can be called propagation, but if it be considered 

 in the following way, it will be clear, I think, that it is properly 

 a propagation, though not a fructification. Supposing the two 

 original lobes to be called A, the second B, the third C, and so 

 on, the mother frond will be represented by AA, the second by 

 AB BA, the third by AC CB BC CA, of which CB BC have 

 no part of the original frond, and may, therefore, be considered 

 as entirely new individuals. Another mode of increase is from 

 the swarming of the grains of the endochrome, which become 

 individualised as in other Algse, and so give rise to a new gen- 

 eration. These bodies are figured with filiform appendages by 

 Braun* in Pediastrum granulatum. But, besides these modes 

 of propagation, there is another, respecting which we shall have 

 * Algarum luiicellulariuiu genera nova et minus cognita, tab. 2 B. 



