PHOTO STUDY 16-1 Pinus (pine): T.s. section of the stem 2 year
At the outside, recognize the cork. In the primary cortex, see numerous resin canals, each lined by an epithelium of secreting cells. Many cortical cells are deeply stained by reason of their tannin and/or resin contents. Identify the phloem. Most of it is secondary, as is indicated by the radial alignment of its cells. Identify apparently empty sieve cells, and the densely stained phloem parenchyma cells.
PHOTO STUDY 16-2 Pinus (pine): T.s. section of a 3 year stem
Starting with the xylem and proceeding outward, see how the older phloem tissue has been crushed until the sieve cells have been literally flattened, though they are still in rows. The outermost phloem tissue is primary, and it has been crushed the most.
PHOTO STUDY 16-3 Pinus (pine): T.s. section of a 6 year stem
By the 6th year, often all semblance of cells has been lost. Recognize the phloem rays. Identify the cambial zone, three to five layers of thin-walled cells lying between xylem and phloem. In the xylem, note the three annual rings.
PHOTO STUDY 16-4 Pinus (pine): T.s. 6 year stem, HP
See how spring tracheids and summer tracheids differ from each other in diameter, in wall thickness, and in location of pits. Work out the detailed structure of sectioned tracheids. Distinguish middle lamella and secondary walls. Are pits equally abundant on all walls, or only on certain ones? Are there any pits at all in summer tracheids? Do you find any pits on tangential walls in summer tracheids?
PHOTO STUDY 16-5 Pinus (pine): T.s. 3 year stem near pith
How much of the xylem is secondary in this photo? Identify protoxylem and metaxylem. Is primary xylem a continuous band around the pith at this level: Why, or why not? Throughout the secondary xylem, you will find more resin canals. Has this tissue any connection with parenchyma rays in the xylem?
PHOTO STUDY 16-6 Pinus (pine): T.s. 4 year stem near pith
From this 4-year stem, you may be able to recognize the irregular outline of the pith, made from primary xylem bundles. Note also where the cambium started the activity of producing secondary xylem. When did activity start to lay down secondary xylem from the original cambium? Spring or summer? How can you tell?
PHOTO STUDY 16-7 Pinus (pine): radial section of young stem. LP
In the transverse section, you saw xylem rays in their edge-view, but here you will see them is face view. How extensive is a ray in a vertical direction? Are all rays of the same height? The long, slender cells with their axes perpendicular to the rays are tracheids. Note furthermore that tracheid ends overlap one another. Distinguish between spring and summer tracheids, and again note differences between them in the size and frequency. Recognize a resin canal in this sheath of parenchyma.
PHOTO STUDY 16-8 Pinus (pine): radial section of young stem. HP
Discover that, in most rays only the middle portion consists of parenchyma cells. Along the top of the ray, cells are generally tracheid-like. A few tracheids, associated with the rays lie perpendicular to the main tracheids of the stem. These are called ray tracheids. One ray tracheid can be seen atop the parenchyma ray. Again make out the detailed structure of tracheids, ray tracheids, and the ray itself, particularly with respect to pits. In bordered pits, note the border, the pit aperture, and the outline of the torus. Where a ray is apparently crossed by tracheids, see that the latter have only simple pits. What kind of pits do you find in ray tracheids? Note the large nuclei in ray cells.
PHOTO STUDY 16-9 Pinus (pine): tangential section of xylem and phloem.
Note the shape of tracheids in this view (red stained cells), and also the location of their pits. A tangential section of the stem gives you an end-view of the rays. How many cells in height are they? How may cells in width? Where do you find bordered pits, and where simple pits? What wall are pits present in the tracheids? The green stained cells are the secondary phloem. Where tracheids are produced in xylem by cambium, sieve cells are produced in the phloem.
PHOTO STUDY 16-10 Pinus (pine): Tang. L.s. primary and secondary phloem LP
The large “rectangular” cells of phloem, which seem to be empty, are sieve cells, and the large, rounded ones with heavily stained contents are phloem parenchyma cells. Are sieve cells actually empty? Do phloem cells retain their protoplasm at maturity?
Do phloem cells have primary walls only, or both primary and secondary walls? Are phloem ray cells similar to, or different form, xylem cells? Compare primary and secondary phloem with regard to regularity of cells, cell types, and prevalence of ray cells.
PHOTO STUDY 16-11 Quercus (oak): Xylem tissue. T.s.stem Spring summer
Note the conspicuous annual rings and the distribution of vessels. Is this wood ring porous or diffuse porous? Are there vessels in the late summer wood? The smaller xylem cells, empty and relatively thin walled in the spring growth are tracheids.
PHOTO STUDY 16-11B Quercus (oak): Xylem tissue. T.s.stem with tyloses.
Note the inordinately large cells, compared to the rest of the cells in the xylem. Also note within these vessels are balloon looking sacs of varying sizes. These are the protoplasmic extensions of vasicentric parenchyma that extended through pits between the parenchyma and the xylem vessels, due to a differential in osmotic pressure when the vessel becomes inactive. These are tyloses.
PHOTO STUDY 16-12 Tilia (basswood): Xylem tissue. T.s. stem
Note the annual rings in the secondary xylem, and in them, identify the vessels. Is this wood ring-porous, or diffuse-porous? Are vessels clustered, or in chains, or isolated? Are vessel walls lignified? Is wood parenchyma terminal, vasi-centric, or diffuse? Are there both uniseriate and multiseriate rays? Most of the other xylem cells are tracheids, though fiber-tracheids are present in the very late summer wood, and there are occasional libriform fibers.
PHOTO STUDY 16-13 Fraxinus (ash): Phloem tissue. T.s. stem. HP
See the variety of cells in the phloem, and note the banded character of the tissue; bands of phloem fibers alternating with bands of the walled phloem cells. Are phloem fibers lignified? Note that as the bands of alternating phloem tissue narrows toward the periphery, the bands of phloem parenchyma ‘rays’ between these ‘bundles’ broaden. How did the widening of such rays occur?
PHOTO STUDY 16-14 Tilia (basswood): T.s. stem cambial zone HP
Identify the cambial zone especially at the level where the parenchyma broadens between “bundles of banded phloem. Notice how rays are produced centripetally in the xylem and centrifugally in the phloem. Follow the phloem ray toward the periphery of the stem. Does the cambium produce wider or narrower phloem rays as it lays down new tissue?
