Methods involving controlled temperature and illumination, continuously renewed sterile culture atmosphere of defined gaseous composition and vapour pressure, and automatic replacement of water loss from the medium have been applied to the analysis of sporophore form in Collybia velutipes and Polyporus brumalis. In P. brumalis progressive suppression of the pileus but enhanced stipe elongation occurs as light intensity (over a certain range) or transpirational water-loss are reduced. These factors act additively. Cap expansion in C. velutipes requires light and is, in addition, inhibited by an atmospheric staling agent removable by KOH and replaceable by CO2. Data and observations on conditions affecting times of production, numbers and dimensions of fruit-bodies are given, especially for P. brumalis. Other aeration factors are considered but shown to have no morphological effect.