This reduction in cellular organelles allows

ribosomes and a vacuole. This reduction in cellular organelles allows nutrients to pass
through the cells more easily. Companion cells, are found adjacent to sieve-tube
elements, and connected to them by plasmodesmata . The nucleus and ribosomes of the
companion cell serve not only the companion cell, but also the adjacent sieve-tube
element. Companion cells may also aid in moving substances into and out of the sap.
Gravity assists in the downward movement of sap. However pressure gradients ,
between the sites of sap production and the sites where sap is unloaded or used, also
moves the fluids either upward or downward in the plant. As sugars are produced in the
photosynthetic leaves ( sugar source ), companion cells pump them into the sieve tubes.
2
Figure 2. Coleus stem tip showing the apical meristem and cells that will give rise to
the three tissue systems including the protoderm, procambium and ground meristem.

Activity #2 Examination of phloem
Observe the slides showing a cross section and longitudinal section of phloem in a stem.
Locate a sieve-tube element, companion cell, sieve-plate and p-proteins . P-
proteins in the sieve tube elements aggregate when the phloem is mechanically
damaged; and act as a “plug” or clot” to prevent leakage of the phloem contents.
Part III. Structure and function of xylem
The upward movement of water in a plant occurs in opposition to gravity but does not
require energy on the part of the plant. Loss of water into the atmosphere, when the
stomata of the plant leaves are open drives the movement of water up the xylem. This
water loss is called transpiration . Xylem is the main water conducting tissue of vascular
3
Figure 3. Cross section through phloem showing sieve-tube elements and companion
cells
Figure 4. Longitudinal section through phloem showing sieve-tube elements and
sieve plates.
Companion
cell
Sieve-tube element showing sieve
plate
Sieve-tube
element
p-protein forming a plug at sieve
plate