a stalk of wheat, or sprig of chia (salvia hispanica), or something similar (see Molina)
Alonso de Molina:
centlaquechcuitl. vna espiga de trigo, de chia, o de cosa semejante. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 18r. col. 1. Thanks to Joe Campbell for providing the transcription.