Family Closteriaceae A.Pritch

Family Closteriaceae A.Pritch.

Cells omniradiate, more or less fusiform and usually arched to a greater or lesser degree. Cell wall with a preformed site of division, whether or not bounded by girdle bands. Cell wall pores are pres- ent but only in the outer cell wall layer, light-microscopically often hard to observe. Possible cell wall sculpture in the form of longitudinal striae (grooves or ridges). Chloroplasts axial, in cross- section stellate, at the cell ends usually bounded by a vacuole containing one or more crystals exhibiting Brownian movement.

Introduction to the Closteriaceae

In Europe the family Closteriaceae involves a single genus, Closterium, that in the lowlands is rep- resented by some sixty species. Some of them are characteristically shaped, others are hard to dis- tinguish from related species. In a number of them the taxonomic status is even questionable as they appear to be mutually connected by intermediate forms. A relevant parameter in identifica- tion is the method of cell wall formation at cell division. Referring to this, two groups may be dis- tinguished: species without girdle bands and species characterized by girdle bands.

Fig. 9. Cell division in a Closterium species without girdle bands (schematically): 0 = germling cell; 1 = daughter cells; 2-4 = three subsequent generations; RW = ring- wall; CB = cross-band (breadth of cross-band largely exaggerated). After Rieth, 1961.

31 DESMIDS OF THE LOWLANDS

In the first-mentioned group of species, the cell wall consists of only two symmetric segments (apart from one or more possible, narrow cross bands in the very midregion). The pattern of cell wall formation in a series of successive generations is given in Fig. 9. A germling cell arising from a zygospore does not have cross-bands. During vegetative cell division the semicells separate at a preformed place wich in a number of species is marked by a ring-wall on the inner side of the wall. Such a ring-wall is newly formed in every successive generation, always at the base of the younger semicell, giving rise to a variable number of cross-bands at the semicell base.

In the other group of species the cell wall consists, in addition to the two apical segments, of one or two rather long, cylindric intermediate segments, so-called girdle bands (whether or not in combination with the above-discussed, narrow cross bands, see Fig. 10). Before the germling (arisen from a zygospore) divides, there is an elongation of the cell by incorporation of a girdle band between the two apical segments when separating at the site of the ring-wall. The girdle band in question develops a new, median ring-wall. When, subsequently, the germling cell divides at the place of that ring-wall, each of the daughter cells gets half of the parental girdle band. From this first daughter generation on, ring-wall formation is consistently at the base of the cell wall segment that is last formed. If this last-formed segment is an apical segment, a ring-wall will indi- cate the place where, as next step in cell development, a new girdle band will be incorporated. The ring-wall subsequently formed at the base of that latter girdle band then indicates the place where

Fig. 10. Cell division in a Closterium species with girdle bands (schematically): 0 = germling cell; 1 = daughter cells; 2-3 = two subsequent generations; RW = ring- wall; CB = cross-band (breadth of cross-band largely exaggerated); GB = girdle band. After Rieth, 1961.