## 3 Discussion  
Results of SEM analysis in _C. praecox_ show that nectaries were distributed on the adaxial side of the innermost and middle petals but not on the abaxial side. No necrary was detected in all five stages (FB, DP, OF, LB, and WP) on the outer petals, including in the receptacle, perianth, stamen, and pistil. The surface morphology of the innermost and middle glands of _C. praecox_ is similar to that of  
Figure 8: Quantitative real-time PCR analysis of different genes in the upper and lower halves of middle petals in DP, OF, and WP stages. _Tublin_ homologous gene of _C. praecox_ was used as an internal control. “um” and “lm” represent upper and lower half of middle petals, respectively. Figure 7: Quantitative real-time PCR analysis of different genes in three different rounds of petals in OF stages. Inner., mid., and out. represent innermost, middle, and outer petals, respectively. _Tublin_ homologous gene of _C. praecox_ was used as aninternal control. (Notes: \(t\)-test used for significant difference analysis; data is the means of relative expression; a, b, c show \(p<0.05\) significant level). the inner petal glands of _Alphonsea glandulosa_ and _Petunia_[16; 28]. The surface of the nectar glands is different from the surrounding epidermis, and nectar stomata are found across the surface of the glandular tissues [28]. The nectar stomata are raised slightly above the epidermis with an aperture for nectar secretion [29]. Although the locations of nectaries within flowers vary highly in terms of broader taxonomic terms, their locations are constant at the family level [30]. Nectaries are usually associated with carpels and stamens in eudicots, but are related to perianth in basal angiosperms [19]. _C. praecox_ belongs to Calycanthaceae, Laurales, Magnoliids, and is clustered to Magnoliales, Piperales, and Canellales, which are close to Chloranthales, Austrobaileyles, Nymphaeales, and Amborellales [31]. In Magnoliidae, _C. praecox_ has no necary distribution on its receptacle, perianth, stamen, or pistil, but has some on the adaxial side of the innermost and middle petals; this finding partly supports that of a previous study, nectaries position within flowers trends to shift from peripheral perianth in basal taxa to central reproductive organs in more derived taxa [19; 20]. Monoterpenes, such as myrcene, geraniol, linalool and sesquiterpene compounds, are the main aroma components of _C. praecox_[32; 33]. Therefore, the concentration of universal precursor of monoterpene (GPP) and its substrate IPP can indirectly reflect the aroma production of _C. praecox_. qPCR analysis of _CpIPP_ and _CpGPPS_ gene in different parts of petals can indirectly determine the location of aroma substances. The expression levels of the necary development related genes _CpCRC_, the other four YABBY family genes (_CpFIL_, _CpYABBY2_, and _CpYABBY5-1/2_) and five floral scent genes (_CpIPP_, _CpGPPS1_, _CpGPP_, _CpLIS_, and _CpTER5_) in the innermost petals of _C. praecox_ were higher than those in the middle and outer petals, but they were the lowest in the outer petals. The _CpLIS_ expression was increased seven-fold at the OF stage, which is responsible for \(\alpha\)-linalool biosynthesis [27]; and \(\alpha\)-linalool accounts for 36% of the total quantity of volatile compounds has been reported in wintersweet flowers [34]. The expression results were consistent with the characteristics of necary distribution based on SEM analysis (Figures 1 and 7). The expression pattern of five YABBY genes in _C. praecox_ by qPCR was in accordance with the RNA-Seq in DP, OF, and WP stages (Figures 5 and 6). The expression levels of _CpIPP_ and _CpGPPS_ were significantly different in the different halves of the petals; those in the upper halves were significantly lower than those in the lower halves during the first two periods.