Frustrated

Hello,
I am trying to add up the diameters of a series of stems on a group of trees. here's how i'm getting the data from our data logger:
Plot Plant Stem No. Diameter
1 1 1 5
1 1 2 5
1 1 3 2
1 2 1 4
1 2 2 5
2 1 1 10
2 1 2 1
2 1 3 5
2 1 4 4

etc. there are a variable number of stems per plant, so i've been trying to figure out how i can "tell" excel to sum a series of cells until it reaches the end of that plant, then go on to the next plant eventually ending with:
Plot Plant DiaSum
1 1 15
1 2 10
etc.

I feel this should be easy, but I can't seem to figure it out. If anyone can point me in the right direction, i would appreciate it.