LESS. XVI 11 MYCELIUM 185 



discs, circular or nearly so in outline, and distinctly thicker 

 in the centre than towards the edge : they float on the fluid 

 so that their upper surfaces are dry. Each of these patches 

 is a young Penicillium-growth, formed, as will be seen 

 hereafter, by the germination of a group of spores. 



As the growths are examined day by day they are found 

 to increase steadily in size, and as they do so to become 

 thicker and thicker in the middle : their growth is evidently 

 centrifugal. The thicker central portion acquires a fluffy 

 appearance, and, by the time the growth has attained a 

 diameter of about 4 or 5 mm., a further conspicuous change 

 takes place : the centre of the patch acquires a pale blue 

 tint, the circumference still remaining pure white. When 

 the diameter has increased to about 6-10 mm. the colour of 

 the centre gradually changes to dull sage-green : around this 

 is a ring of light blue, and finally an outer circle of white. 

 In all probability some of the growths, several of which will 

 most likely occur in the saucer, will by this time be found 

 to have come together by their edges : they then become 

 completely interwoven, their original boundaries remaining 

 evident for some time by their white tint. Sooner or later,, 

 however, the white is replaced by blue and the blue by sage- 

 green, until the whole surface of the fluid is covered by a 

 single growth of a uniform green colour. 



Even when they are not more than 2-3 mm. in diameter 

 the growths are strong enough to be lifted up from the fluid, 

 and are easily seen under a low power to be formed of a 

 tough, felt-like substance, the myceliuvi (Fig, 40, a my\ from 

 the upper surface of which delicate threads, the aerial 

 hypha {a. hy.\ grow vertically upwards into the air, while 

 from its lower surface similar but shorter threads, the sub- 

 vierged hyfh(P. {s. hy.), hang vertically downwards into the 

 fluid. 



