In desmids, asexual reproduction takes place by simple cell division. In the first instance it is the nucleus, situated in the centre of the isthmus, that multiplies (a process which is hardly or not to be observed light microscopically as it requires fixation and staining with specific dyes).
Following this pocess, between the doughter nuclei a new cell wall, as a transverse septum in the lengthened isthmus, is constituted. Subsequently, the istmial compartments separated by this septum gradually grow to new semicells. The mutual contact between the two developing, young semicells is maintained for a long time so that cell division results in formation of a couple of doughter cells which initially, by way of their young semicells, are still linked to each other.
Cell division in Cosmarium biretum, shown in 9 steps
1. Vegetative cell before division. Each of both semicells exhibits a couple of large chloroplasts provided with lamellae that radiate from a centrally located pyrenoid.
2. When the nucleus multiplies, the isthmus (bridging the two semicells) is somewhat lengthened.
3. After doubling of the nulear mass a new cell wall between the doughter nuclei is constituted (septum in the isthmus).
4. At the septum the lengthened isthmus starts to constrict.
5. Constriction of the isthmus is about half-way across. Initiation of young semicells.
6. Young semicells develop as expanding buds from the parent semicells.
7. The new semicells come to their final shape but cell wall is still thin and without any sculpture.
8. The newly formed semicells are almost full-grown, species-specific granulation of the cell wall is already visible.
9. Cell division is completed. In the near future doughter cells will be separated.
All 9 images © Henk Schulp
From the above-shown series of images it is clear that growth of new semicells not only goes with increase of volume but also with morphological differentiation, both internal and external. This is even better illustrated by morphologically highly differentiated desmid species, such as representatives of the genus Micrasterias.
Some stages of cell division in Micrasterias rotata
1. Early stage of division: the new semicells are still in the phase of bud-like expansions.
2/3. First beginning of morphological differentiation: initiation of an apical lobe with on either side two lateral lobes.
4. Onset of splitting up of the upper lateral lobes of the first order into lateral lobes of the second order.
5. In addition to lateral lobes of the first and the second order, in the upper part of the semicell also an onset of differentiation of lateral lobes of the third order can be seen.
6. Lateral lobes of the first, second and third order are well to be distinguished. However, morphological differentiation of the young semicell is not yet completed for lateral lobes of the fourth order are still wanting.
All 6 images © Henk Schulp