## 2 Results  
### Distribution Characteristics of Nectary on Petals of Different Stages  
SEM analysis was performed to determine the distribution characteristics of necary on petals in different stages of petals in different developmental stages of floral meristem (FB, DP, OF, LB, and WP), and receptacle, pistil, and stamen (Figure 1A,F,K,R,W). The nectaries were mainly located in the adaxial side of the innermost and middle petals (red asterisk shown, Figure 1B-E,G-J,L-O,S-V,X-Z2). The adaxial/abaxial side of the outer petals and the abaxial side of innermost/middle petals of different stages had no necary distribution. Figure 1P,Q show the adaxial and abaxial sides of the outer and middle petals in the OF stages (in green box), respectively, which there is no necary detected at all. The numbers of nectaries changed in the FB to WP during flower senescence. The numbers of nectaries were equal or slightly higher in the middle petal than that in the innermost petal under the same magnification (\(\times\)400, Figure 1). Morphological difference in different stages of nectaries are shown in Figure 1. With the development of floral meristem, the length-width ratio of nectaries became small and evaginated (\(\times\)1800, Figure 1). The substance of floral scent was found beside the nectaries (red arrow shown, Figure 1S,Z1,Z2). The cell size increased from FB and DP to OF with the development of floral meristem. This phenomenon is similar to that in LB and WP. Larger number of nectaries was observed in FB or DP than in OF at the same magnification (\(\times\)400) because of the different cell sizes in different development stages (Figure 1B,D,G,I,L,N). The number of nectaries in OF was higher than that in LB or WP (Figure 1L,N,S,U,X,Z1), with similar cell size in three different development stages.  
No nectary was detected on the receptacle, perianth, stamen, and pistilin all five stages (FB, DP, OF, LB, and WP) (just show results in OF stage, Figure 2). Concentrated nectaries were found in the region near the axis of the lower part of petals, but almost none in the upper and edge of  
Figure 1: Morphology of the surface of petal glands on different stages in the _Chimounthus praecox_ by SEM. (**A**,**F**,**K**,**R**,**W**)_** floral meristem of five different developmental stages (FB, DP, OF, LB, and WP); (**B**,**C**,**G**,**H**;**L**,**M**;**S**,**T**,**X**,**Y**) nects on adaxial side of innermost petals in five different developmental stages. (**D**,**E**,**J**;**N**,**O**,**U**,**V**;**Z1**,**Z2**) nectaries on the adaxial side of middle petals in five different developmental stages. (**B**,**D**,**G**,**I**,**L**,N**,S**,U**,**X**,**Z1**) surface of the glandular tissue, showing the nectary stomata (red asterisk). (**C**,**E**,**H**,**J**,**M**,**O**,**T**,**V**,**Y**,**Z**) close-up of the nectary stomata. (**P**,**Q**) (in green box) no nectary distribution on adaxial side of outer petal and abaxial side of middle petals in OF stages. Scale bar is the same in (**A**,**F**,**K**,**R**,**W**); (**B**,**D**,**G**,**J**,**L**,N**,P**,**Q**,**S**,U**,**X**,**Z1**); and (**C**,**E**,**H**,**J**,**M**,**O**,T**,**V**,**Y**,**Z**), respectively. Red arrow shows the substance of floral scent. The magnification was \(\times\)400 (left) and \(\times\)1800 (right), respectively.  
petals (Figure 3 just show the stages of FB and OF). Nectaries are not uniformly distributed in the petals (Figures 1B,D,G,I,L,N,S,U,X,Z1 and 3). These distribution characteristics of nectaries were perhaps related to the \(YABBY\) gene family, which controls the build of nectaries development and dorsiventral polarity.  
Figure 2: No nectaries distribution of the stamen and pistil in OF in the _Chimonanthus praecox_ by SEM. (**A**), receptacle; (**B**), perianth; (**C**), front of stamen; (**D**), back of stamen; and, (**E**), pistil.