In Jain food rules, lotus stem (कमल काकड़ी / bhee) is generally considered not Jain.

Reason in simple words:

• Lotus stem grows underwater in mud, like a root / underground part.
• When it is taken out, a lot of small lives (निगोद / सूक्ष्म जीव) in mud and around the root are harmed.
• Such items are treated like root / anant-kay dravya, which strict Jains avoid (similar to potato, onion, etc.).

So:

• Strict Shwetambar and Digambar Jains – usually avoid lotus stem.
• Some modern families – may eat it, but from the point of view of ideal Jain acharan, it is better to avoid.

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