Multicellularity and polarity are usually directly linked. Nondirectional growth by division is rare in the plant kingdom. In the simplest case, multicellular organisms come in to being, because the daughter cells that exist due to the division of a mother cell stay together. If the single cells differentiate, i.e. they specialize, then it is spoken of a cell colony. The differentiation is morphologically visible, if the single cells differ from each other by features like size, pigment composition, etc. Often nevertheless, the differences are based purely on complicated biochemical traits that can only be recognized under laboratory conditions or by different behaviors. A differentiation of cells within a colony and within a multicellular organism requires a communication of the cells with each other. One cell, for example, can secrete a certain substance that is recognized by its neighboring cell as a signal and triggers this cell into changing its physiological activity. Those parts of the cell wall that are now between neighboring cells loose their contact to the extern environment and can develop new functions like durable cell-cell contacts.
Most primitive, multicellular cells are thread-like. Depending on the species, elongation of the thread, i.e. the filament, occurs either by division of the terminal cell or by division of any cell within the filament, a modus called intercalary division. In intercalary division, the nuclear spindle is in parallel to the filament’s axis, while the metaphase plate is perpendicular to it. The largest part of the cell wall of each cell remains in contact with the extern environment and only the end walls serve to make contact with the neighboring cells.
The development of almost sphere-like cell aggregations occurs only in rather few species like Pandorina, Eudorina, Gonium, Pediastrum, etc. Their aggregations do rarely contain more than 16 cells. In organizations of higher numbers, the single cell has less and less contact with its extern environment and thus also with the supply of nutriments. As long as no efficient supply network exists – and this developed only in higher plants – compact multicellular colonies have hardly any advantages but quite some heavy disadvantages. Ramified filaments and two-dimensional structures form a kind of intermediary type of organization between filaments and compact colonies. Both are caused by changing orientations of the spindle of division.
The development of hollow spheres as in the case of the Volvocales is a special case. The cells of one colony exchange information even though they remain largely autonomous. A colony reacts always as a whole with a movement towards light.
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