Reducing agents must be combined with other chemicals to confine their activity to the silver grains that have been exposed, to control the rate of reaction, ...
We have already pointed out that a change in tone associated with a change in the composition of the developer is caused by a change in the size and shape of the silver grains that form during the development.
As far as I know in the b&w film the silver grains get exposed depending on (i.e. the image intensity depends upon) the light intensity arriving on them and therefore depending on the % reflectance of each original colour.
These crystals contain silver grains that determine how sensitive the film is to light exposure, and how fine or grainy the negative the print will look.
These silver grains are typically microscopic, but they can clump together in certain films or under certain development conditions. When they clump to the extent that they become visible, the picture appears grainy.
-Grains - exposed and developed silver halides which have formed black metallic silver grains, producing the visible photographic image.
0648 gram; (3) fine textured appearance of a negative, print, or transparency resulting from the clumping of silver grains. (4) the smallest component of a photographic image. A single particle of silver or dye cloud.
 See also: Back, Photograph, Film, Image, Silver
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