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PHOTO UNIT 9:
VASCULAR SYSTEM: PHLOEM
OBJECTIVES:
1. Characterize phloem in general both structurally and
functionally.
2. Characterize sieve elements and sieve cells, functionally and
structurally, using at least the following structures:
sieve plates (simple vs. compound), sieve areas,
walls, and protoplasts.
3. Distinguish between slime plugs and callose and discuss the
significance of callose in sieve tube function.
4. Describe the companion cell, phloem parenchyma and phloem
fibers in terms of origin, characteristics, function,
and spatial location in the phloem.
5. Interpret sectioned bundles of plant stems as it relates to
phloem cell types.
Photos for this study:
9-1: Cucurbita (squash): t.s. stem vascular bundle, phloem tissue
9-2: Cucurbita (squash): l.s. stem vascular bundle phloem tissue
9-3: Ibervilla t.s. stem phloem
PHOTO STUDY 9-1 Cucurbita (squash): t.s. stem vascular bundle, phloem tissue
Recognize the sieve tubes, which are of large diameter and seemingly empty. Are they really empty, or do they merely seem to be so because of large central vacuoles? Is there a protoplast in the sieve tube cell? Do you find nuclei in any of them? Note that the walls are slightly thickened, compared with those of parenchyma cells. Are these walls lignified? Alongside each sieve tube just above the large central element, identify the small companion cell, each with a dense protoplast. Would you expect to find nuclei in any of the companion cells? Note in the center of the photo an end wall of a sieve tube element in face view. Here you see the conspicuous pits that give the end wall its sieve-like appearance. It is called the sieve plate. PHOTO STUDY 9-2 Cucurbita (squash): l.s. stem vascular bundle phloem tissue
Sieve tubes can be recognized by their vertical alignment, wide diameter, and horizontal sieve plates with deep pits. In older sieve tubes, slime material has accumulated against the end walls, and it extends from there back through a sieve tube cell in a funnel-shaped mass. The slime is proteinaceous. It arose as minute slime bodies, appearing as droplets through the cytoplasm. These bodies, which may be distinguished in the younger tube cells, become confluent and then coagulate to form the central, funnel-shaped strands. When this slime accumulates on the sieve plates and dips down into the coarse pits, it is called a slime plug. INTERPRETIVE PHOTO STUDY
9-3: Ibervilla t.s. stem phloem
Identify all the phloem cell types shown in this photo.
PHOTO STUDY 9-1 Cucurbita (squash): t.s. stem vascular bundle, phloem tissue
Recognize the sieve tubes, which are of large diameter and seemingly empty. Are they really empty, or do they merely seem to be so because of large central vacuoles? Is there a protoplast in the sieve tube cell? Do you find nuclei in any of them? Note that the walls are slightly thickened, compared with those of parenchyma cells. Are these walls lignified? Alongside each sieve tube just above the large central element, identify the small companion cell, each with a dense protoplast. Would you expect to find nuclei in any of the companion cells? Note in the center of the photo an end wall of a sieve tube element in face view. Here you see the conspicuous pits that give the end wall its sieve-like appearance. It is called the sieve plate.
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