OF THE PEDIASTEE^. ^9 



in close apposition and form a globular frond. The cells in the typical species, 

 S. echinatiun, are 8 or 16 in number, and so arranged that all their apices 

 converge towards and meet in the centre of the frond. 



SjyJicerodesmus, which probably is rightly accounted one of the Pediastreae, 

 is named, but not described, by Niigeli ; we are, however, informed by Braun 

 (Gen. Nova, p. 70, in foot-note) that its fronds are composed of 4 spherical 

 cells closely aggregated in a rhomboidal form. 



Development and Growth. — Seenedesmus multiplies by fission, as Ealfs 

 believed, in an obhque direction, but according to Nageli, parallel to the long 

 diameter of the cells. The former adopted his opinion from the features of 

 biserial chains ; but the latter interprets those appearances by the simulta- 

 neous occurrence of longitudinal and of transverse fission (I. 37, 39). 



The process of self- division commences generally at the same period in 

 each cell of the frond (family, Niigeh), and proceeds with so great rapidity 

 that its intermediate stages are unobserved. One of the two terminal cells 

 (I. 39), or, in an eight-celled, probably the two, sometimes remain for awhile 

 undivided. The cell separates into two, then each of these again into two 

 others, and at times this act of subdivision is repeated a third time. By a 

 more prolonged act of segmentation of the cell-contents, the result is a number 

 of minute cells which arrange themselves in rows of two, four, or eight, and 

 thus form miniature fronds which ultimately escape the parent-cell by rup- 

 tui'e. Occasionally, adds Niigeli, the young fi'onds are connected together 

 by mucus, formed by dissolution of the parent cell-wall. Development takes 

 place in parallel planes, although by their increase they become mutually 

 compressed and in-egular, and the chains curved j)rior to their discharge. 



This production of macrogonidia and their cohesion into fronds has not 

 been seen by Braun, and is^ in his experience, an exceptional phenomenon 

 {Gen. Nova, p. 67). 



When Mr. Ealfs wrote his work on the Desmidieae (in 1848), he had to 

 confess himself altogether ignorant of the modes of reproduction both of 

 Pediastrum and Seenedesmus. He, however, described self-di\ision of the 

 cells in both genera, but rightly regarded this process as one not of develop- 

 ment, but of vegetative increase and repetition. On this subject he remarked 

 {op. cit. p. 182), — " I have not seen the cells duiing the process of division, 

 but I am informed by M. de Brebisson that it takes place at the notch, in 

 the same manner as in other Desmidieae : hence the cells in each cii'cle are 

 connected at their ends, like those of the filamentous genera. I do not, 

 however, understand in what manner the additional circles are formed, nor 

 why the numbers in each circle are so constant." 



Niigeli, likewise, was equally ignorant of the propagation of Pediastrum, 

 but thought it highly probable it resembled that of Seenedesmus. The num- 

 ber of cells in a table or frond, indicated to his mind its production by a series 

 of fissions in the power of two ; and he presumed that a new frond was gene- 

 rated within a parent cell by division of its protoplasm, just as in Seenedesmus, 

 — a supposition supported by the fact that the entire young fronds are not 

 larger than the single cells of mature specimens, that like these they are 

 composed of the same number of individual cells similarly disposed, and 

 undergo no subsequent segmentation into a larger number. 



The more recent researches of Braun are confii^matory of the views of Na- 

 geli (Gen. Nova, p. 68). Amid a large number of specimens of Pediastrum 

 Boryanum he detected the escape of, in most instances 32, more seldom 

 of 16, and rarely of 8 gonidia, from a parent cell, — the number gene- 

 rally, but not invariably, corresponding with that of the cells composing the 

 matui-e frond or coenobium. The collection of gonidia was enclosed by a 



