 If it occurs before sex chromosomes become heteromorphic, as is likely in the octoploid red sorrel "Rumex acetosella", sex is determined in a single XY system.In a more complicated system, the sandalwood species "Viscum fischeri" has X1X1X2X2 chromosomes in females, and X1X2Y chromosomes in males.Non-vascular plants

Ferns and lycophytes have bisexual gametophytes and so there is no evidence for sex chromosomes.