Development and structure of the phloem tissue

Conclusion

The modem morphological concept of the phloem is that of a tissue containing highly specialized elongated enucleate cells with permeable cytoplasm arranged in longitudinal rows and forming anastomosing strands together with the parenchymatous members of the phloem. These specialized cells—that is, the sieve-tube elements or their counterparts—are intimately connected with each other. In most plants these connections are localized in wall areas occupied by the sieve-plates or by similar structures comparable to highly developed simple-pits. The connecting strands, although varied in size, are larger than ordinary plasmodesmata and are usually associated with variable amounts of callose in the wall region that they traverse. The parenchyma cells show different degrees of ontogenetic and physiologic relation to the sieve-tube elements. The companion-cells of angiosperms are most closely associated. with the sieve-tube elements; then follow the albuminous cells of gymnosperms and finally the phloem parenchyma and ray cells. When the sieve-tube reaches maturity its protoplast is in a premortal state. The element is, therefore, rather short-lived. The companion and albuminous cells die at the same time as the sievetubes.