Observation on the structure change of phloem cells in the developing wheat caryopsis

The structure change of phloem cells in developing wheat( Tritium aestivum. L ) caryopsis was systematically investigated via optical microscopy, scan electron microscopy and transmission electron microscopy. The results showed that the main vascular bundle did not appear to be fully differentiated at the 7 th day after anthesis. The fully differentiated sieve elements could be observed at the 12 th day after anthesis. The estimated number of sieve elements was about forty and they distributed as semi circular at the outside of vascular bundle. From 14 th to 20 th after anthesis, the SEs were enriched with mitochondria, P type plastid, and many microfibril like structures. Some SEs had Nacreous Wall. Some SEs had typical CC and SE/CC complex could be formed. CCs contained electron dense protoplasm and nucleus. Numerous plasmodesmata were found between intermediary cell and phloem parenchyma cell; or between phloem parenchyma cells. There were plasmodesmata zones existed between phloem parenchyma cells. From 24 th to 29 th day after anthesis, the number of mitochondria decreased in SEs, CCs and phloem parenchyma cells contained electron light protoplasm, these means that their transport function were decreased. Intermediary cells still contained numerous mitochondria and other cell organelles. Intermediary cells had same structure as SE when the protoplasm of intermediary cell was pushed to all around of cell cavity. So intermediary cells were some special parenchyma cells that suited to photoassimilate transport. It had an importance function during later filling stage. The 39 th after anthesis, phloem cells became feeble and deformed; wheat caryopsis developed mature.