OF THE PEDIASTEE^. 25 



gulaiiy quadiilateral, but their free margin is more or less deeply notched, 

 and therefore bilobed (I. 44, 45, 53, 62). The lobes are usually tapering, 

 and form a tubular process either truncate or acute at the extremity (I. 62). 

 In a few cases the notch is not angular, but cui-ved and crescentic (I. 62) ; 

 in others again it is deep, angular, and gaping (I. 52, II. 27), and gives the 

 cell an irregular figure ; this latter condition is more seen where only a few 

 cells are united together, and where the lobes are not prolonged as pro- 

 cesses. In some species, moreover, the lobes are terminated by short hair- 

 like spines. 



The notch on one side is not confined to the peripheral ceUs, but extends, 

 in several species, also to the contained cells of the frond (I. 52, 66) ; their 

 lobes, however, are not tapering, but sharply truncate. Nageli instituted a 

 subgenus of Pediastrmn under the name of Anomopedium, the chief charac- 

 teristic of which is the absence of bilobed peripheral ceUs (I. 46, 47, 48). 



The ceUs of Ccelastmm are hexangular (I. 49, 50, 51), the central ones 

 very regularly so, whilst the peripheral are rounded oif on their free aspect 

 in one species, and in another notched and bilobed (I. 54, 55). Those, lastly, 

 of Sorastrum (I. 56, 57) are wedge-shaped or triangular, with rounded 

 angles ; they cohere by their apices, whilst the base is perij^heral, often 

 rather concave or emarginate, and, as a rule, armed at each angle by a pair of 

 short spines. On a lateral view the cells are oblong (I. 58). 



There is a pervading uniformity in the contents of the cells of the different 

 genera of Pediastreae, which consist of the usual vegetable protoplasm, and 

 are spoken of collectively as the endochrome. At first the colour is very pale 

 green, but it becomes deeper with advancing age, and in fully matiu'e or 

 decaying cells is seen to change to red or brownish red, just as the leaves of 

 trees change colour on the approach of autumn. At first, the protoplasm is 

 clear and homogeneous, but in course of time granules appear, enlarge in 

 bulk, and multiply in number. Moreover, each cell presents a single ehlo- 

 rophyU-vesicle, which is least discernible in very yoimg and in very old cells 

 (I. 53 to 58). It is ordinarily seen in or about the centre of the cell, but 

 may occur on one side, as in Pediastrum Rotula. Around this vesicle are 

 seen in several species clear cii^cular spaces or globules, recalling those of 

 Closterium, varying in number in difierent cases from two to six (I. 44, 45, 

 53). In Pediastmm Rotula, Nageli observed two such ; in P. Boryanum and 

 P. Selencea (I. 44, 45), from two to six ; in the species of Scenedesmus and 

 of So7%istrum (I. 57), one hyaline space. This author likewise represents the 

 relative position of the chlorophyU-vesicle and of the translucent space to be 

 constant in similar fronds. In those made up of two cells only, the chloro- 

 phyU-vesicle is placed outside, whilst the clear cavity lies against the parti- 

 tion-wall. In chains of four to eight cells the chlorophyll- vesicle is external 

 relatively to the central cell, and the clear space internal (I. 40, 41), — the 

 position being regulated, not by the partition -wall, as in the Palmelleae in 

 general, but by the centre of the entire frond. Oil-globules are also con- 

 tained in the cells ; their presence is readily demonstrated by the addition of 

 tincture of iodine, for they continue colouiiess when the surrounding mass is 

 coloured brown ; their position often exhibits much regularity. Unless the 

 chlorophyll- vesicle be esteemed nuclear, no nucleus has been cUscemed in the 

 cells of Pediastreae. On one occasion Nageli saw, in Pediastrum Boryanum, the 

 endochr^ome disposed in a radiating manner around the chlorophyll-vesicle, 

 an arrangement which often obtains in Algse, and in many vegetable ceUs 

 where there is a central nucleus. 



T^UMBER AND DISPOSITION OF THE CeLLS IN THE FllONDS. The CcUs of Pc- 



diastrese are always united together in compound fronds. The number so 



