  We examine the evolution of Li in the Galactic disk,basing our model on the
compilation of Li v.Fe observations in stars of iron abundance ranging between
halo values,which give a baseline for the disk Li abundance,and 0.2 dex above
solar.The upper envelope of these observations is taken to represent the locus
of minimum stellar Li depletion,and thus to follow the Galactic IS Li
abundance.The most striking feature of this envelope is its steep rise between
Fe/H ~ -0.4 and -0.2,corresponding to a relatively late epoch.The Li abundance
increases by an order of magnitude,from its halo value of logLi ~2.2 to the
"current" value of ~3.2 within this narrow range of Fe abundances.It is well
known that spallation reactions between CNO and 4He in the ISM produce Li,but
models give a nearly tenfold shortfall for this process.The alpha+alpha fusion
reaction appeared to yield too much early Li or too little current Li.These
failures led to the exploration of stellar sources:supernovae,novae,and the He
flash phase in AGB stars.The Li-Fe envelope is a strong constraint on any
process.Our models can account for this in the context of disk chemical
evolution with a near constant or growing rate of low metallicity gas
infall,and a purely interstellar Li source: alpha-alpha fusion.The Li
production rate is found to be proportional to the gas expulsion rate from
intermediate and low mass stars.Low-energy alphas,emitted by these stars,but
accelerated in more energetic processes associated with SNe,or the bow-shocks
of stellar winds,can yield the observed abundance and its variation with Fe or
O.Our model is consistent with the cosmic ray spectrum in the few MeV
range,corrected for solar modulation and with new 7Li/6Li ratios found by
Knauth et al.(2000) in the local ISM.
