-vessel elements are the building blocks of the water transportation system of the plants. They are responsible for the ascent of sap from roots to leaves and the tip of the plants. These are lignified cells which are surrounded by parenchymal cells. These cells vary in length. The vessel members originate from longitudinal stacks of cells. All the vessels are connected to each other at ends through perforated, tube-like structures. The perforations or the perforated plates may also vary in shape. conducting elements of angiosperms, wide, efficient in water conduction.
-Tracheids are the elongated cells of the xylem. They also lack protoplasm like the vessels. They are similar to vessels except for a few facts. Tracheids originate from individual cells. These cells are imperforate and have only pit pairs on the common walls. A tracheid's build varies by location. Tracheids are present in all vascular plants. Main conducting element gymnosperms and pteridophytes, originate from single cells, and large pits. Not that efficient in conduction but is safe.
Sieve cells- elongated cells with tapered end walls, have no companion cells, sieve areas do not form sieve plates, sieve areas are not well differentiated , found in pteridophytes and gymnosperms.
Sieve tubes- consist of vertical cells placed one above another forming long tubes connected at the end walls by sieve pores, have companion cells, areas confined of sieve plates, sieve areas are well differentiated, found in angiosperms.
Joseph S. Levine, Kenneth R. Miller
1st EditionJanet L. Hopson, Postlethwait
Johnson, Peter H. Raven