On what basis did all scientists attribute cyanobacteria to plants for a very long time, in particular to algae, and only at the end of the 20th

On what basis did all scientists attribute cyanobacteria to plants for a very long time, in particular to algae, and only at the end of the 20th century. were they placed in the realm of bacteria?

The relatively large sizes of the cells (the nostrum, for example, forms rather large colonies that can even be picked up), carry out photosynthesis with the release of oxygen in a manner similar to higher plants, and the external similarity with algae was the reason for their consideration earlier in the composition of plants (“blue-green algae “).
And at the end of the twentieth century it was proved that cells of blue-green nuclei do not have, and chlorophyll in their cells is not the same as in plants, but characteristic of bacteria. Now cyanobacteria are among the most complex and morphologically differentiated prokaryotic microorganisms.

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