From the introduction:
"In the New Check List (1960) a number of genera of mainly white-spored agarics containing taxa without or with lateral or excentric stems (Figs 1-3) were included in the family Pleurotaceae. It was realised then that this could only be a temporary measure pending further studies on the affinities of the genera included. This part of the Flora describes these agarics together with a few others without or with lateral stems, and attempts to show their distribution according to more recent studies. A few mainly lignicolous taxa with central stems are also included because of their affinities to the genera included. In the Introduction to this Flora (Henderson, Orton & Watling, 1969) the genera were largely defined and listed according to the 1960 list. Many new genera appear in this part of the Flora."