Phenomena of protoplasm components' selective autolyzing in the development of secondary phloem sieve elements in Populus deltoides

To do some researches on the physiological function of sieve tube elements(SE) in secondary phloem,it is key to observe the changes of plasm components during the development of SE.Using electronic microscopy,the development of secondary phloem SE was investigated in Populus deltoides.According to the changes of plasm components and cell structure,the development process of SE in secondary phloem could be divided into three stages,ie immature,mature and degenerated stages.The immature stage pointed to the development of SE before vacuole membrane was ruptured.In this stage,the cell radial expanding,cell wall incrassation,sieve element plastid and P-protein producing of SE occurred.The mature stage started when the vacuole membrane was ruptured.Whereafter,the protoplasm of SE autolyzed selectively,and the mature SE had being formed.In mature stage,the organelle,such as dissociative ribosome,Golgi body,endoplasmic reticulum and nuclear,was disassembled quickly.The nucleus was degenerated dissimilarly in two ways:1) the nuclear membrane was degenerated earlier,and karyoplasm joined with P-protein was dispersed;2) the chromatin was agglomerated earlier.Subsequently,the karyotin was almost degenerated,but the nuclear membrane still kept two layers structure clearly.The disassembled plasm membrane of SE was a sign in the degenerating stage.Meanwhile,plastid membrane was disorganized and the starch grain was dispersed in the chamber of SE.In addition,the mitochondria was dissembled at last.Synchronously,the callose appeared in sieve plate,and P-protein was disaggregated.Later on,the opened sieve plate was formed because of callose autolyzing after the protoplasm disappeared completely.Ultimately,the physiological function of SE was lost.