CONFEEVOIDE^E. 



[ 198 ] CONFERVOIDE^E. 



lacse. They are composed of linear series 

 of cells, mostly inflated so as to give the 

 filaments a beaded appearance ; the linear 

 series increase in length by transverse divi- 

 sion, and in some stages subdivide longitu- 

 dinally; larger globular cells (sporanges) 

 occur at intervals in the lines, with others 

 devoid of endochrome (vesicular cells, 

 Thwaites). During the increase, the older 

 external membranes soften into a gelatinous 

 coat. In Nostoc, where the filaments accu- 

 mulate in large numbers, they lie elegantly 

 curled and entwined in masses of this jelly, 

 which exhibit a more or less definite, lobed, 

 external form ; appearing to the naked eye 

 as gelatinous crusts or globular masses, 

 as they lie upon damp ground or among 

 mosses. Each sporange produces one 

 resting-spore, which breaks out from it in 

 germination. 



Nearly allied to Protococcus stands a 

 family which until recently have often been 

 regarded as animals, namely the Volvoci- 

 nese ; which consist essentially of groups of I 

 i organisms identical with the ciliated zoo- 

 spores, held together in a definite form by a 

 common membranous envelope or cceno- 

 bium, through which the cilia penetrate, so 

 that the entire full-grown plant moves free- 

 ly in the water, as in Volvox, Gonium, Pan- 



j-spores, 

 fecundation by spermatozoids. 



The Desmidiacese form another tribe of 

 very simple organization, where the indivi- 

 dual plant is composed of a single cell ; but 

 here the coat or enclosing membrane is pe- 

 culiarly characterized by the assumption of 

 remarkable forms unlike anv other vegetable 

 structures ; presentingangular and escalloped 

 outlines or elegant processes projecting from 

 the wall, but always so as to exhibit a bi- 

 lateral symmetry. These cells are isolated 

 or arranged in linear series or beautiful 

 complicated star-like groups, enclosed at 

 first in a common gelatinous envelope, but 

 readily breaking up into isolated frustules. 

 They are further remarkable for exhibiting 

 the process of conjugation with great dis- 

 tinctness; resulting in the production of 

 peculiarly formed bodies, zygospores, with 

 rigid external membranes, which are 

 generally regarded, probably correctly, as 

 sporanges. They are also reproduced by 

 zoospores. 



The DiatomacesD are nearly related in 

 many respects to the Desmidiaceae ; but, on 



the other hand, diverge from the ordinary 

 characters of plants so much in other re- 

 spects, that some authors have placed them 

 in the animal kingdom. Like the Desmi- 

 diacese, they are microscopic simple cells, 

 isolated or coherent in groups ; and either 

 free, or imbedded in a definitely or indefi- 

 nitely formed mucous nidus. They differ 

 however from the Desmidiacese, in possess- 

 ing, when free, a more active power of loco- 

 motion, and also by being often attached 

 by a kind of foot, and this either shigly or 

 in large polypiform families. Their great 

 distinctive character is the presence of a 

 siliceous coat to the cell, which preserves 

 the form of the organism when the soft parts 

 are removed by fire or acids. The cell- 

 contents of the Diatomaceae are usually of 

 a dirty yellow colour, the colour of the 

 chlorphyll being concealed by a yellow 

 matter diatomine (phycoxanthine). The 

 reproduction is by division, and by conju- 

 gation, analogous to that of the " Desmi- 

 diacese. 



The Oscillatoriaceae are truly filamentous 

 plants, the component parts of which, though 

 readily separating under external influences, 

 are often combined into complex fronds in 

 their normal state. The filaments of this 

 group are mostly very minute, and exhibit 

 transverse markings, which in some cases 

 are so delicate that they cannot be regarded 

 as actual divisions of cell-contents by septa j 

 yet the filaments break readily across in 

 these places, and the fragments go on grow- 

 ing. In the larger forms the articulations 

 of the cell-contents are more distinct ; but 

 even here the filaments look like rows of 

 individual masses of cell-contents contained 

 in a common tube, forming a kind of sheath. 

 In some genera the filaments are contained 

 in bundles in secondary sheaths. The most 

 remarkable point about this tribe is the 

 occurrence of the peculiar kind of motion 

 in the typical genus Oscillatoria, whence 

 it derives its name: the filaments emerge 

 readily from their sheaths and wave back- 

 wards and forwards, and the broken frag- 

 ments oscillate like the beam of a balance ; 

 from what cause, or by what means, is still 

 undecided. 



The only known mode of reproduction is 

 by the breaking up of the filaments into 

 longer or shorter pieces, or into single joints. 

 Peculiar large cells occur at the base of the 

 filaments of some of the adnate genera ; but 

 their nature is unexplained. 



The Siphonacese are plants of larger 



