Parenchyma cells of secondary phloem in Pinus strobus.

Parenchyma cells of the secondary phloem in Pinus strobus have all the cellular organelles common in other plant cells. They have mitochondria, endoplasmic reticulum, ribosomes, dictyosomes, and plastids. Parenchyma cells are very conspicuous because of their organic inclusions, starch and lipids. Plasmodesmata in transverse and tangential walls of axial parenchyma cells and in end walls of ray parenchyma cells are regularly distributed and of uniform size, about 500 Å in diameter. In radial walls of axial parenchyma cells and horizontal walls of ray parenchyma cells plasmodesmata are located in primary pit-fields; there they are of variable size and often divided into several branches. The branches are confluent into a median nodule. Perforation of the transverse wall between two axial parenchyma cells and the resultant union of the cellular material of the two connected cells is reported.