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245	1	0	_aAnother View of the Ultrastructure of Cucurbita Phloem
490	0		_vAnnals of botany, 30(4), p.563-585, 1966
520	3		_aThe primary phloem of young internodes of Cucurbita maxima was studied with the electron microscope. Phloem parenchyma cells are highly vacuolated and contain nuclei, endoplasmic reticulum, ribosomes, mitochondria, chloroplasts, and occasional dictyosomes. As compared with parenchyma cells, the most distinctive features of companion cells are their extremely dense cytoplasm, low degree of vacuolation, lack of chloroplasts, and numerous sieve-element connexions. Companion cells contain plastids with few internal membranes. At maturity the enucleate sieve element is lined by a plasmalemma, one or more cistema-like layers of endoplasmic reticulum, and a membrane which apparently delimits the parietal layer of cytoplasm from a large central cavity. In OsO<- and glutaraldehyde-fixed elements, the central cavity is traversed by numerous strands, which run from cell to cell through the pores of sieve plates and lateral sieve areas, and which are derived ontogenetically from the slime bodies of immature cells. Numerous normal-appearing mitochondria are present in the parietal layer of cytoplasm. The pores of sieve plates and lateral sieve areas are lined with cytoplasm. The ultrastructural details of young sieve elements differ little from those of other young nucleate cells. During sieve-element development, the sieve element increases in vacuolation. At the same time, slime bodies develop in the cytoplasm. With glutaraldehyde fixation, these bodies often exhibit a double-layered limiting membrane. As the sieve element continues to differentiate, the slime bodies increase in size and the parietal layer of cytoplasm becomes very narrow. Presently, the slime bodies begin to disperse and their contents fuse. This phenomenon occurs in the parietal layer of cytoplasm, while the latter is still delimited from the large central vacuole by a distinct tonoplast. The initiation of slime-body dispersal more or less coincides with perforation of the pore sites, and many pores are traversed by slime early in their development. Before slime-body dispersal, all dictyosomes and associated vesicles disappear from the cytoplasm. Eventually, the tonoplast diappears and the slime becomes distributed throughout the central cavity in the form of strands. Nuclei and ribosomes disappear before breakdown of the tonoplast. Sieve elements are connected with companion cells and parenchyma cells by plasmodesmata.
700	1	2	_aEvert, R.F.
700	1	2	_aMurmanis, L.
700	1	2	_aSachs, I.B.
856	4	0	_uhttps://drive.google.com/file/d/1cDtnoNEjaX2-uZNF1WEO2Wi0fOmA8_HX/view?usp=drivesdk _zPara ver el documento ingresa a Google con tu cuenta: @cicy.edu.mx
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