▁the ▁Agricult ural ▁School ▁at ▁B uit enz org . ▁From ▁ 1 9 0 5 ▁till ▁ 1 9 2 4 ▁Back er ▁was ▁employed ▁at ▁the ▁botan ical ▁gard ens . ▁He ▁sc oured ▁the ▁fields ▁and ▁for ests ▁of ▁Java , ▁Mad ura , ▁and ▁the ▁adjacent ▁K ange an ▁Islands , ▁and ▁so ▁collected ▁about ▁ 3 0 , 0 0 0 ▁spec im ens ▁while ▁making ▁daily ▁tri ps ▁of ▁thirty ▁to ▁forty ▁kilometres ▁on ▁foot . ▁ ▁Pas ur uan ▁ ▁From ▁ 1 9 2 4 ▁till ▁ 1 9 3 1 ▁Back er ▁was ▁employed ▁at ▁the ▁‘ Pro ef station ▁voor ▁de ▁Java - Su iker indust rie ’ ▁( Ex perimental ▁Station ▁for ▁the ▁sugar ▁industry ▁of ▁Java ) ▁in ▁Pas ur uan ▁on ▁East ▁Java . ▁He ▁wrote ▁a ▁fl ora ▁on ▁the ▁we eds ▁of ▁the ▁can e ▁fields ▁of ▁Java ▁( 7 5 3 ▁species ). ▁ ▁The ▁problem ▁of ▁vol cano ▁Krak ata o ▁ ▁Mel ch ior ▁Tre ub ▁sent ▁Back er ▁on ▁two ▁exped itions ▁to ▁the ▁vol cano ▁Krak ata o , ▁in ▁ 1 9 0 6 ▁and ▁in ▁ 1 9 0 8 . ▁In ▁ 1 9 2 9 ▁Back er ▁published ▁his ▁content ious ▁book ▁The ▁problem ▁of ▁Krak ata o , ▁as ▁seen ▁by ▁a ▁botan ist , ▁in ▁which ▁he ▁maintained ▁that ▁not ▁all ▁plant ▁life ▁had ▁been ▁destroyed ▁by ▁the ▁gig antic ▁er u ption ▁of ▁the ▁vol cano