"Consist in" vs. "consist of"

Of is the correct preposition to use in your second example:

The body consists of cells.

In is the correct preposition to use in your first example:

Meditation consists in attentive watchfulness.

Consist of means to be composed or made up of, while consist in means:

To have the thing mentioned as the only or most important part.
Tolerance consists in respecting other people’s opinions.

I think both the above answers are correct, but missing a key point that 'consist of' is normally used with more tangible objects whereas 'consist in' has an esoteric quality to it. There's a good explanation here about the difference:

American writers often ignore this distinction. Consist of is used in reference to materials; it precedes the physical elements that compose a tangible thing. The well-worn example is that concrete consists of sand, gravel, cement, and water.

Consist in (= to have as its essence) refers to abstract elements or qualities or Intangible things. Thus, a good moral character consists in integrity, decency, fairness, and compassion. ...

This construction is literary in tone and is not often seen today in general writing. Sad to say, it may now seem creaky to most readers.

(Bryan Garner, Modern American Usage (3rd ed 2009), quoted in Language Usage Weblog October 1, 2009)

I have to agree about the 'creaky' bit. I didn't even know it was legitimate to say "consist in" until I saw this question, and the examples in the earlier answers sound really strange to my ears.

Consist of X means made up of X, while consist in X means have X as essential feature. The meaning is slightly different. For example,

His breakfast consists of noodles, eggs and bananas.

Her music consists in inspiring others.

One would write

Meditation consists in attentive watchfulness

to mean that watchfulness is an essential feature of meditation and

The body consists of cells

to mean that cells make up the body.