Fungi are eukaryotes: their cells contain nuclei and organelles both of which are enclosed by membranes. Yeasts differ from other forms of fungi both structurally and in their means of reproducing. Multicellular fungi differ from yeast in that they grow in the form of a system of tubes which are capable of growth and branch formation. Research has been carried out into fungal colonization of concrete the results of which are useful in understanding how damage is done to concrete and also how this damage might be prevented. Some fungus display optimal growth rates under alkaline conditions. Nutrients required in very small quantities by fungi include calcium, iron, copper, manganese, zinc and molybdenum. A number of concrete specimens underwent expansion and cracking: Aspergillus niger, Aspergillus versicolor, Cladosporium cladosporioides and Aspergillus flavipes. Visual inspection indicated that Aspergillus niger caused the most substantial deterioration.