Anatomy of two mechanisms of breaking physical dormancy by experimental treatments in seeds of two North American Rhus species (Anacardiaceae)

Anatomy of the endocarp was studied in relation to the physical dormancy-br eaking mechanisms in experimentally treated Rhus aromatica var. aromatica a nd R. glabra germination units, which include seed plus endocarp (hereafter seeds). The endocarp has three distinct layers, with brachysclereids on th e outside, osteosclereids in the middle, and macrosclereids on the inside. Brachysclereids in the carpellary micropyle region (i.e., region immediatel y adjacent to the integumentary micropyle) are shorter than those in other parts of the endocarp. and the macrosclereids in this region are not elonga ted. Thus, a weak point is formed in the endocarp. Concentrated sulfuric ac id broke seed dormancy in R. aromatica by eroding the brachysclereids and o steosclereids in the carpellary micropyle region, whereas boiling water bro ke dormancy in seeds of R. glabra by inducing a blister adjacent to the car pellary micropyle.