Question

why do branches of old trees are different than the stems of new plant​

Answer 1

With respect to tissue, there is a difference in the outer layer of branches of old tree and outer layer of stem of young plant. The old tree has layer of dead cells commonly called bark which mainly consist of cork cambium - The Phellogen.

Answer 2

Answer:

ᴛʜᴇ ʙʀᴀɴᴄʜᴇs ᴏғ ᴏʟᴅ ᴛʀᴇᴇs ᴅɪғғᴇʀᴇɴᴛ ғʀᴏᴍ ᴛʜᴇ sᴛᴇᴍs ᴏғ ᴀ ɴᴇᴡ ᴘʟᴀɴᴛ? ... ᴡɪᴛʜ ʀᴇsᴘᴇᴄᴛ ᴛᴏ ᴛɪssᴜᴇ, ᴛʜᴇʀᴇ ɪs ᴀ ᴅɪғғᴇʀᴇɴᴄᴇ ɪɴ ᴛʜᴇ ᴏᴜᴛᴇʀ ʟᴀʏᴇʀ ᴏғ ʙʀᴀɴᴄʜᴇs ᴏғ ᴏʟᴅ ᴛʀᴇᴇ ᴀɴᴅ ᴏᴜᴛᴇʀ ʟᴀʏᴇʀ ᴏғ sᴛᴇᴍ ᴏғ ʏᴏᴜɴɢ ᴘʟᴀɴᴛ. ᴛʜᴇ ᴏʟᴅ ᴛʀᴇᴇ ʜᴀs ʟᴀʏᴇʀ ᴏғ ᴅᴇᴀᴅ ᴄᴇʟʟs ᴄᴏᴍᴍᴏɴʟʏ ᴄᴀʟʟᴇᴅ ʙᴀʀᴋ ᴡʜɪᴄʜ ᴍᴀɪɴʟʏ ᴄᴏɴsɪsᴛ ᴏғ ᴄᴏʀᴋ ᴄᴀᴍʙɪᴜᴍ - ᴛʜᴇ ᴘʜᴇʟʟᴏɢᴇɴ