Sentence:  Her Ladyship, in a letter to A. Aikin, Esq., [...] dated Castle Bernard, Ireland, Oct. 19, 1827, states that she has made some improvement in the mode of preparing the rye-straw, which is the material for <WSD>plat</WSD> employed in the school under her ladyship's patronage. 
Meaning of the word marked with <WSD> tags:  Material produced by braiding or interweaving, especially a material of interwoven straw from which straw hats are made.