Results
Our studies showed that Origanum vulgare ssp. gracileforms two types of flowers, perfect and pistillate.
Perfect flowers . Flowers are complete, zygomorphic, and heteromerous: pentamerous in calyx and corolla, tetramerous in androecium and dimerous in gynoecium.
The calyx is gamophyllous, campaniform, with 13 veins, and consists of a tube and a limb. The aestivation of the calyx is valvate. The tube of the calyx is pubescent on the outside with scattered short hairs with an admixture of sessile glands, less often only with punctate glands or glabrous. The inside tube is lined with a ring of hairs, not exserting from the calyx, equal to half of its tube. Teeth of calyx 2–3 times shorter than the tube, almost equal, triangular-lanceolate, acute.
The corolla is bilabiate, white or pale pink, and consists of a tube, a throat, and a limb. The aestivation of the corolla is imbricate. The corolla tube exserts from the calyx tube, densely pubescent on the outside covered by short hairs with an admixture of glands, inside are scattered hairs in the middle part. The upper lip of the corolla is bilobed, sinuate, with wide lobes, directly continuing the corolla tube without forming an angle with it. The lower lip of the corolla is somewhat longer, deviated downwards from the corolla tube, three-lobed, with a slightly larger middle lobe. All three lobes of the lower lip are rounded.
The androecium is tetrameric, haplostemonous, and didymous. The stamens are four, anterior (lower) longer than posterior (upper). All four stamens are fertile. The anterior stamens are longer than the upper lip of the corolla, the posterior stamens are shorter and located in the corolla throat. The stamens are parallel, ascending under the upper lip of the corolla. The filaments adhere to the corolla tube for the most part and are free only in the throat. The anthers are all identical, elliptical, 4-locular, diverging approximately at an angle of 90º from each other, separated by a triangular connector. Anthers are introrse, their sacs are opened by a longitudinal slit.
Pollen grains are 6-sulcate, 3-celled, ellipsoidal or broadly ellipsoidal. Pollen fertility varies from 91.2 to 98.6%.
The gynoecium is dimerous and syncarpous, the ovary is superior, 4-lobed (in the vast majority of flowers), and 4-locular. Ovary lobes are round-ovate or ovate, smooth and glabrous. Style is gynobasic and emerges from the gap between the ovary lobes from their base. The style is longer than the upper lip of the corolla and the anterior stamens. The style is bifid at the top, its stigmas are almost equal. Occasionally there are flowers with an abnormal number of parts of the gynoecium, with three stigmas and six ovary lobes.
A nectariferous disk consists of 4 lobes and is located at the base of the ovary, all its lobes are the same size.
Pistillate flowers . The structure of the calyx, corolla and gynoecium of perfect and pistillate flowers does not differ. The stamens in pistillate flowers are represented by staminodes. The staminodes of pistillate flowers are smaller than the stamens of perfect ones, do not form pollen, and are hidden in the corolla throat (Fig. 2).
Pistillate and perfect flowers differ in the size of their parts (Fig. 4). The size of the calyx and its parts in perfect and pistillate flowers do not differ significantly. However, the width of the teeth of the calyx is always smaller in pistillate flowers than in perfect ones.
Significant size differences between the two types of flowers were found in the corolla and androecium. The petals and staminodes of pistillate flowers are significantly smaller than in perfect ones. However, the two types of flowers do not differ in the length of the upper lip and the width of the middle lobe of the lower lip of the corolla. The length of the calyx/corolla tube ratio is the most notable difference between the two types of flowers. The corolla tube of pistillate flowers barely exceeds the calyx due to the strong shortening. On the contrary, the corolla tube of perfect flowers exserts far from the calyx.
A comparison of the parameters of the gynoecium in perfect and pistillate flowers shows a different nature of size differences. The length of the stigma lobes and the width of the ovary lobes are similar in both types of flowers. In contrast, the length of the ovary lobes is larger in pistillate flowers than in perfect ones.
Ordination of perfect and pistillate flowers . The ordination of perfect and pistillate flowers of O. vulgare ssp. graciledemonstrates the nature of the differences between them (Fig. 5). The PC1 (F1=49.6% variance) is closely related to the traits in which the two types of flowers differ most significantly (length of the corolla tube, length of the lower corolla lip, length of the middle lobe of the lower lip, length of the upper and lower stamens, length and width of the anthers, style length, nectar disk blade length) (Fig. 2). The PC2 (F2=24.7% of the variance) unites the characters by which the two flower types do not differ significantly: the length and width of the calyx teeth, the height of the calyx throat, the length of the upper lip of the corolla, and the width of the middle lobe of the lower lip.
Thus, pistillate and perfect flowers differ quite significantly from each other in regard to several criteria. The most important of these is the size of the corolla and its parts, the ratio of the length of the corolla tube and calyx, the position of the anthers, and the degree of development of the stamens.
Flowering biology . Perfect flowers of Origanum vulgaressp. gracile are characterized by a pronounced protandry: the anthers open even at the stage of buds. At this time, the stigma lobes are folded together, not ripe and still unable to receive pollen. Later, after the anthers are empty, they diverge from each other at an acute angle and become covered with papillae, which indicates their maturation and receptivity to pollen. After the corolla begins to wither, the stigmas take on an anchor-like shape and are covered with pollen.
Our studies have shown that the pollen/ovules ratio in the perfect flowers of O. vulgare ssp. gracile varies from 825 to 953. So, according to R.W. Cruden (1977), this species is facultatively xenogamous.
Sex ratios. O. vulgare ssp. gracile has perfect and pistillate flowers on the different plants. Therefore, O. vulgare ssp. gracile is a gynodioecious species. Hermaphrodites always predominate in the sex ratios; their proportion varies from 58.9% to 76.2% (Table 1).
The ordination of female frequency and environmental parameters (Fig. 6) showed that the first two PC included 84.8% of the variance. The PC1 (F1=56.9%) positively correlate with the frequency of females and annual precipitation (bio12), and correlate negatively with the aridity index. The PC2 (F2=27.9%) has a strong positive correlation with mean annual temperature (bio1) and annual temperature range (bio7), and a moderate negative correlation with female frequency. However, a partial correlation between the frequency of females and annual precipitation is statistically significant (r=0.944, P=0.045), while the same correlation between the proportion of females and mean annual temperature is not statistically significant (r=0.069, P=0.930). Thus, annual precipitation has a stronger effect on female frequency than mean annual temperature.