Parasitic plants are characterized by production of a large number, of one of the smallest seeds in the world's flora, of length less than 1 mm, frequently called ‘dust seeds’. The paper presents detailed data concerning the seed productivity and their size as well as morphological diversity of a very rare and endangered parasitic species Orobanche bartlingii (Orobanchaceae). O. bartlingii occurs from central and eastern Europe to China, although it is very rare and sporadic, totally absent in large parts of Europe. In Poland, O. bartlingii occurs mostly in the Silesia-Cracow Upland, sporadically in the Małopolska Upland and in the Noteć valley. Libanotis pyrenaica is the most frequently reported as the host plant of the Bartling's broomrape. O. bartlingii in Poland prefers thermophilous fringe vegetation of the Geranion sanguinei (Trifolio-Geranietea sanguinei class) and Prunetalia spinosae alliances (Rhamno-Prunetea class), xerothermic grasslands of the Festucetalia valesiacae order, usually with the saxicolous Festucetum pallentis association (Festuco-Brometea class). The studies were conducted at the most abundant population of the species in the Silesia-Cracow Upland (S Poland). The populations of species are located in the Podzamcze near Ogrodzieniec, on the highest elevation of the Silesia-Cracow Upland, the Janowski Mountain (515.5 m).

The working hypothesis of this study was that: (i) Orobanche bartlingii seeds coming from the same ovary differ in size and dimension, (ii) the slope exposure where Orobanche plants grew influences the seed productivity as well as some morphological traits. We examined the diversity of some morphological traits: the length of shoot (A) and corolla tube (B), the number of flowers per shoot (C). The aim of the studies was to show the seed dimensions and individual productivity, i.e. numbers of seeds per fruit (D), percentages of smaller seeds in ovaries (E), total number of seeds per plant (F). We also analyzed the data as well as the correlation between measured traits for all population and independently for plants growing on different slope exposure. The O. bartlingii seeds differ in terms of size and dimension. In each individual we found smaller oval seeds and bigger, more oblong ones. We state that the exposure does not have a significant impact on the (A, B, C, F traits) but individuals from N exposure featured a significantly lower percentage of smaller seeds in ovaries. We noted that more fertile individuals, i.e. those producing more seeds per individual contain smaller number of small seeds. The number of seeds per individual ranged from 1,870 to 627,250 with the coefficient of variation of about 99%. We found that higher individuals were characterized by a greater number of flowers, a higher seed production, a longer corolla tube and a higher number of seeds in the ovary no matter on which slope exposure they grew. Knowledge of the biology of endangered species, especially fertility and morphological diversity in microhabitat is essential for their protection.