The effect of relative humidity on the shape ofBacillus apiarius spores

Bacillus apiarius spores have been examined at relative humidities between 99% and 12% in a scanning electron microscope modified to enable wet specimens to be studied. The spores have also been studied both wet and dry with an interference microscope. Their shape remains rectangular whether wet or dry. A calculation of the effect of an osmotic pressure change of about 200 atm upon the maximum deflection of the longest side of the spore shows that the deflection is less than 4.5 nm. The shape of the spore therefore is not markedly affected by a change from dry to wet and the shape will remain as it was when the coat was initially formed, unless the coat is weakened by some chemical attack. The refractive index of the coat material is 1.532–1.536.