Sentence:  While the lowest units of phytocoenoses, the <WSD>sociations</WSD>, are founded upon a relative homogeneity in "all" layers, most consociations are homogeneous only in one layer, and the higher units in no layer at all, being founded mainly upon the "sociological affinity" of the dominants of the layer arbitrarily chosen as the base for the classification (most of the highest of the layers represented). 
Meaning of the word marked with <WSD> tags:  A plant community.