Both companies confirmed last week that Bayer had launched an offer for the US seeds giant.
Monsanto is primarily known for genetically modified crops, often leading to vocal activist criticism.
The offer comes amid a wave of mergers in the industry.
Rivals DuPont, Dow Chemical, and Syngenta have all announced tie-ups recently, although they have yet to be cleared by regulators.
Bayer said the offer of $122 per share represented a 37% premium on the price of Monsanto shares before rumors about the takeover bid emerged in the media.
When news about the takeover offer broke last week, Bayer shares took an 8% hit and a number of large Bayer investors voiced their criticism of the prospective deal.
Bayer has a market value of about $90 billion, making it the second-largest producer of crop chemicals after Syngenta.
Monsanto, which has a market capitalization of $42 billion, attempted to buy Swiss rival Syngenta in 2015.
However, Syngenta ended up accepting a $43 billion offer from ChemChina in February, although that deal is still being reviewed by regulators in the US.
Bayer’s acquisition of Monsanto is expected to be bigger in value than the ChemChina-Syngenta deal.
The biggest merger in the chemicals industry took place late last year when Dow Chemical teamed up with Du Pont to form a new $130 billion company.
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