28 GENERAL HISTOEY OF THE INFUSOEIA. 



from the same parent individual, and lastly, to remark the analogy presented 

 in other allied genera, e. g. Scenedesmus, Sorastrum, and Coelastrum, to con- 

 vince himself of the fact. 



Moreover, as shown by Nageli, when the number of cells is the same in a 

 frond (coenohium, Braun), their arrangement varies considerably, tending 

 more and more to irregularity as the ceUs are more multiplied. " The normal 

 and most frequent disposition is orbicular, the cells being arranged, according 

 to their number, in one or several concentric circles, around either a single 

 central cell or none at all. Where two cells are placed in the centre, the 

 circles around incline to an elliptic figure; fi^om this a transition to an 

 elongated form still more aberrant from the orbicular type is indicated, in 

 which the elongate -elliptic circles surround several intermediate cells placed 

 in single or double longitudinal series. By the less regular concentric or the 

 entirely confused disposition of the cells, the elliptic form passes at length 

 into others still more abnormal, such as reniform, panduriform, cuneiform, 

 &c., all which agree in ha\ing 64 or 128 ceUs. The regular concentric 

 arrangement is moreover deranged by the occasional intercalation of ceils 

 referable to no one of the circles ; and lastly, owing to an incompleteness 

 of the circles of cells, they become so connected one with another, that a 

 spiral disposition is the result, which, although abnormal in every species, is 

 in some specimens constructed with wonderftd regularity. All these various 

 arrangements arise from the manner in which the motile gonidia are disposed 

 and marshalled in their first stage ; for these are distributed within the 

 parent depressed orbicular cell, according to the laws of juxtaposition, in a 

 plane." 



Another peculiarity in the disposition of the ceUs in the fronds of Pedias- 

 trwn is, that sometimes, instead of being all in juxtaposition, so as to form 

 an unbroken congeries of cells, or, in the language of Nageli (op. cit. p. 94), 

 instead of being parenchymatic, apertures or interspaces are left between 

 them (I. 53). This is most seen where the inner cells are more or less bi- 

 lobed, so that an opening subsists between the lobes of each ceU ; but similar 

 apertures may likewise occur at the angles where the cells come into contact. 

 "When the position of the cells in the table is regular, that of the foramina is 

 so also. Pediastrwn Selencea with 16 cells has, as a rule, 6 large and 8 small 

 openings ; the large are bounded by 3 cells, the small by 2 ; the small spaces 

 are sometimes absent, when the large become very evident. Pronds of the 

 same species, having 32 cells, display usually 11 larger interspaces lying 

 betwixt 3 cells, and 18 smaller enclosed between 2 cells. 



Anomopedium, a subgenus of Pediastrum, differs not only in its peripheral 

 cells not being bilobed, but also in having its cells partially disposed in a double 

 plane (I. 46, 47, 48). The ceUs which are in the numerical series of 4, 8, 

 16, 32, and 64, are subject to manifold arrangements, and frequently aggre- 

 gated quite irregularly. They are mostly so placed, that in one, two, or even 

 three directions, they can be clearly discerned to be in parallel straight rows. 

 A concentric disposition is quite exceptional ; not unfi-equently, instead of aU 

 the cells occupying the same plane, some form a partial second layer upon 

 the other about the middle. 



In Ccelastrum (I. 50, 51, 52) the hexangular cells are so arranged that 

 they form a hollow, globular, areolar frond. Coel. sphcericum consists of 25 

 to 40 cells, which compose a lamina perforated by 3, 4, 5, 6, 8 angular meshes 

 (areolae) somewhat larger than the cells themselves, and from 13 to 22 in 

 number. Coel. cuhicmn consists of 8 cells united in a cubical form, hollow 

 inside : on each side are 4 cells enclosing a quadrangular aperture. 



Lastly, in Sorastrum the wedge-shaped or cordate cells (I. 56, 57) are all 



