[R] vectors levels are carried through to subsets...

[chipmaney]

I have a dataset.  Initially, it has 25 levels for a certain factor,
Description.

However, I then subset it, because I am only interested in 2 of the 25
factors.  When I subset it, I get the following. The vector lists only the
two factors, yet there remain 25 levels:

> Quadrats.df$Description
 [1] Emergent 25x75  Emergent 25x75  Emergent 25x75  Emergent 25x75 
Emergent 25x75  Emergent 25x75  Emergent 25x75  Emergent 25x75  Emergent
25x75 
[10] Emergent 25x75  Emergent 25x75  Emergent 25x75  Emergent 25x75 
Emergent 25x75  Emergent 25x75  Hydroseed 25x75 Hydroseed 25x75 Hydroseed
25x75
[19] Hydroseed 25x75 Hydroseed 25x75 Hydroseed 25x75 Hydroseed 25x75
Hydroseed 25x75 Hydroseed 25x75 Hydroseed 25x75 Hydroseed 25x75 Hydroseed
25x75
[28] Hydroseed 25x75 Hydroseed 25x75 Hydroseed 25x75 Hydroseed 25x75
25 Levels: Black Cottonwood Black Cottonwood Enhanced Emergent Emergent
25x75 Floodplain 1 Floodplain 2 Floodplain 3 Hydroseed 25x75 ... Western Red
Cedar Enhanced

This seems rather innocuous; however, when I run a by statement, it returns
a list with 25 entries, 23 of which are of course NA....is there a way to
avoid this?

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