Euglenophyta

In these algae the interphase nucleus lies in the central or posterior region of the cell; it is spherical in most spindle-shaped species, and ovoid or long and narrow in elongate cells. In the largest species the nucleus is at least 30 x 15 µm in dimensions, but in small species the spherical nucleus may be less than 2 µm in diameter. The chromosomes retain their condensed condition throughout interphase, appearing as granular or filamentous threads (Figure 2.81a and 2.81b). In some nuclei the chromosomes radiate from the central endosome, while in others (even in the same species) they coil haphazardly throughout the nucleoplasm. Mitosis begins with a forward migration of the nucleus so that it comes to lie immediately posterior to the reservoir. In species with several endosomes in the interphase nucleus, these usually fuse to form a single body. The endosome then elongates along the division axis, perpendicular to the long axis of the cell, and the chromosomes orient into the metaphase position, following three main types of orientation:

• Pairs of chromatids from late prophase orient into a circlet of single chromatids, separation and segregation having occurred during orientation (Egracilis)
• Pairs of chromatids from interphase or prophase come to lie along the division axis still as pairs, parallel to one another and to the elongated endosomes (Euglena communis, Euglena viridis)
• Single chromosomes from prophase line up along the division axis and there undergo duplication into the pairs of chromatids of that mitosis (Eacus, Euglena spirogyra)

These different types overlap to a certain extent, species differ mainly in the time at which the double structure of the chromosomes first becomes microscopically visible. In all cases, the endosome continues to elongate and the chromatids segregate towards the ends of the endosome into two daughter groups. This stage, with most but not quite all of the daughter chromosomes separated, must be called metaphase in Euglena. During this early-to-late metaphase succession, the locomotor apparatus (flagella, photoreceptor, and eyespot) replicates and the reservoir divides. The daughter reservoirs open into the still single canal, but each now has its own contractile vacuole, eyespot, and flagella. Separation, segregation, and anaphasic movements of the chromatids are irregular, and this, coupled with a very low chromatid velocity, results in an extremely long anaphase. The end of the naphase is marked by a sudden flow to the poles of the central region of the elongated endosome, and the persistent nuclear envelope seals around the groups of chromatids and the daughter endosomes to form the telophase nuclei. Once telophase is established, with separate daughter nuclei, one of the two flagella in each daughter reservoir grows to emerge as a locomotory flagellum. A cleavage line is initiated between the now distinct daughter canals and progress helically backward to separate the daughter cells. High chromosome numbers are the rule for species of Euglenophyta, indicating a possible polyploidy.

Chlorophyta
Nuclear and cell division has been intensively and extensively studied in the green algae at lightmicroscopic and ultrastructural level, and several different patterns have been recognized. Two basic patterns have merged from these studies regarding nuclear and cell division in green algae:

• Intranuclear mitosis, with the nuclear envelope closed at metaphase or open at the poles, interrupted by the microtubules of the spindle; other microtubules, transverse to the longitudinal axis of the spindle, are present at telophase and are called the phycoplast; the latter functions somehow in cytokinesis, by furrowing or by cell-plate formation; the daughter nuclei at telophase are in close proximity
• The spindle and nuclear envelope are open and a phycoplast is absent; at telophase the microtubules of the intranuclear spindle are persistent and a phragmoplast-like structure is organized as cytokinesis by furrowing proceeds

Cylindrocapsa is an example of the first type; the parietal chloroplast divides before mitosis. Two pairs of centrioles are already present at the beginning of the interphase. In early prophase, the nuclear envelope is surrounded by one or two layers of endoplasmic reticulum, which is rough, that is, covered by ribosomes. Perinuclear microtubules appear around the nucleus and in late prophase they proliferate within the nucleus to form a tilted mitotic spindle between the pairs of centrioles lying at the spindle poles. At metaphase the nuclear envelope is still intact and surrounded by the endoplasmic reticulum, so that mitosis is closed. The fully condensed chromosomes become aligned to form a distinct metaphase plate and have plate-like layered kinetochores. The nonpersistent telophase spindle soon degenerates, though a few microtubules can still be found around the reformed nuclear enevlopes. The pairs of centrioles migrate around the telophase nuclei, away from the former spindle poles and towards the center of the equatorial plane, where they remain until after cytokinesis.

Cisternae of endoplasmic reticulum proliferate in the narrow zone of cytoplasm present between the two daughter nuclei at the center of the cell. They bleb off smooth endoplasmic reticulum vesicles, which become aligned in the equatorial plane to form a cell plate of smooth vesicles. These vesicles coalesce to form a transverse system separating the daughter cells. Here the vesicles accumulate within a phycoplast, that is, a plate of microtubules lying in the future plan of division. After completion of the transverse septum and the resultant separation of the daughter cells, a new cell wall is secreted around each daughter protoplast by exocytosis of Golgi-derived vesicles containing wall material. Each daughter cell thus gains a complete new wall; in the case of Cylindrocapsa, daughter cells remain united to form filaments, because the parental walls are persistent. In the case of other green algae with this type of mitosis and cytokinesis, daughter cells are liberated from the parent cell wall as non-flagellate autospores, or as zoospores with centrioles ready to form flagella.

Coleochaete possesses the second type of nuclear and cell division; the chloroplast begins to cleave at prophase; the single centriolar pair present during the interphase replicates at prophase and each of the two pairs takes up a position at one pole of the future spindle. Microtubules then form between the centriolar pairs, outside the envelope of the elongate prophase nucleus. The mitosis is open, because the nuclear envelope breaks down during metaphase; endoplasmic reticulum vesicles are present among the spindle microtubules. The chromosomes align in a distinct metaphase plate but the chromosomal microtubules do not attach to defined kinetochores. At early telophase daughter nuclei become separated through elongation of the spindle, which is persistent during telophase and holds the nuclei far apart.

By the end of the telophase, new microtubules proliferate and surround the spindle microtubules, forming the phragmoplast, which includes also actin filaments. Golgi-derived vesicles appear within the phragmoplast, guided by the microtubules and the filaments to the future plane of division, where they become arranged to form a cell plate. The vesicles contain cell wall material and their coalescence produces a transverse septum consisting of two cell membranes with the new transverse wall between them. As the coalescence is not complete, connections leading to plasmodesmata are left between the daughter cells. This type of mitosis and cytokinesis is rare in the green algae, but is the common mode of division in plants such as bryophytes and tracheophytes.