332 Conjugate 



branching are known, usually limited to short lateral outgrowths consisting 

 of a few cells. Such rudiments of branches have been observed in Zygnema 1 

 and Mougeotia (W. & G. S. W., '98; Pascher, '07). 



The cell-wall consists mostly of cellulose in one continuous piece, with its 

 outer surface more or less thickly clothed with mucus, the latter attaining 

 its maximum development in Zygnema anomalum, in which it forms a con- 

 spicuous sheath four times the diameter of the filament. This outer mucous 

 coat sometimes shows evidence of a fibrillar structure perpendicular to the 

 cell-wall, although the latter has no pores as in so many of the Desmidiacese. 



In Spirogyra the protoplast consists of a thin lining layer of cytoplasm and 

 a small centrally placed mass in which the nucleus is lodged, the two being 

 connected by protoplasmic strands which are frequently branched. The 

 approximately central position of the nucleus is, however, a constant feature 

 throughout all members of the family and the general character of the proto- 

 plasmic strands traversing the great sap-vacuole depends upon the form and 

 disposition of the chloroplasts in the various genera. The nucleus may be 

 globular, ellipsoid or lenticular (even in different species of the same genus), 

 and when complanate the flattened sides are always parallel to the plane of 

 division in Spirogyra and parallel to the plane of the chloroplast in Mougeotia. 

 It has, as a rule, one comparatively large nucleolus, but two, or even three, 

 nucleoli may sometimes be observed. 



The most striking feature of all the Alga3 belonging to the Zygnemaceas 

 is furnished by their chloroplasts. These are either axile or parietal, plate- 

 like and solitary, as in Mougeotia, star-shaped and binate, as in Zygnema 2 , or 

 disposed as spiral bands, as in Spirogyra. 



In many of the Zygnemaceas numerous refractive globules of small size 

 often occur in the cells. These have been shown to contain much tannin and 

 are termed tannin-vesicles. 



Rhizoid-like organs of attachment (haptera) are of frequent occurrence in 

 the young plants of a few species of Spirogyra and Mougeotia, but have not 

 been noticed in any of the other genera of the Zygnemacere. Spirogyra 

 adnata and Sp. fluviatilis are usually attached to a substratum, and Delf ('13) 

 has recently given a description of the attaching organs of a species which 

 may be identified with Sp. adnata. The character of the attaching organs 

 varies much in the different species in which they have been found. Some- 

 times they are simple rhizoidal outgrowths, but more often they are branched, 



1 Although but rarely observed, branches have been known for a long time to occur in 

 Zygnema ericetorum ; they may consist of ten or even fifteen cells. 



In a small form of this species, described from the West Indies as ' Z. pachydermum var. 

 confervoides ' (W. & G. S. W., '94), longitudinal septa of an incomplete character have been 

 observed. 



- The only exception is Zygnema ericetorum. Consult p. 346. 



