Acacia Boards That Stand the Way They Grew
Yalkut Shimoni on Torah 371:2
It was taught: "acacia wood, standing up" (verse 15), meaning that they stand in the manner of their growth.
תַּנְיָא. (פסוק טו) עֲצֵי שִׁטִּים עֹמְדִים, שֶׁעוֹמְדִין דֶּרֶךְ גְּדִילָתָן.