The inner fruit structure in Epilobium hirsutum and E. angustifolium was studied under the light microscopy with the aim to find out new taxonomically significant characters for the genus Epilobium. It was revealed that the studied species have different intensity of longitudinal fruit growth and fruit pubescence: in E. hirsutum fruit is covered with unicellular glandular blunt trichomes of various length while in E. angustifolium it is densely covered with falcate tapering trichomes. The pericarpium is composed of 9–13 cell layers in the facets, and it is thickened in ridges. Exocarpium and endocarpium are one-layered and not lignified. Mesocarpium is composed of tissue with thick non-lignified walls, as also chlorenchyma with some aerial cavities and one to six inner cell layers of the mechanical tissue with lignified cell walls. Fruit wall is supplied by four large septal veins and four dorsal veins, ovule vascular supply is trans-septal. Fruit dehiscence is proceeded through two types of longitudinal fissures located along the dorsal veins and in the middle distance of septa. In the process of dehiscence four T-form lobes with distal portion of septa disjoin from the central column. Central column with proximal portions of septa attached retain unlignified in the fruit center till fall down. The fruit in E. hirsutum and E. angustifolium is defined as the inferior dorsal-septifragal tetralocular many-seeded capsule with synascidiate and symplicate zones. Histogenetical pericarpium structure corresponds to the capsule of Forsythia-type.
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