Finally, the 'tertiary semi uniform' characters are sometimes constant and sometimes variable (unifoliate or polyfoliate calyx, number of cells in the fruit, position of ﬂowers and leaves, nature of the stem): with these semi uniform characters it is not possible to define families or orders not because they are not capable, if applied to all the species, of forming general entities, but because they do not concern what is essential in a group of living beings.