[R] t.tests by unique groupes

[Imri]

My data (slength) look like this:
Plant Block Treat Genotype Source MPH
1       1     1     w   05-AZM sp 160            NA
2       2     1     w    12-50    463            NA
3       3     1     w    12-51 sp 150     0.0508982
4       5     1     w    29-42    567     0.9017094
5       6     1     w  KNG-KNG    811            NA
6       7     1     w    02-02      1            NA

Treat column has w or d for each Plant(row). I wont to do t.test of
MPH~Treat for each of the genotypes (128 total). I tried :

t.tests<-by(slength, slength[,4], function(x)
t.test(slength[,6]~slength[,3])) 

but it is not what I need.
Do you have any ideas?
  Thanks
         Imri
 
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