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PHOTO UNIT 8: VASCULAR SYSTEM: XYLEM
OBJECTIVES:
1.
Define complex and simple tissues, name them and distinguish one
from the other.
2.
Characterize mature tracheids, vessel elements and fibers, in terms of
shape, wall thickenings, and pit types.
3.
Discuss
wall thickening types as they relate to function and development.
4.
Discuss end
wall formation and breakdown and classify perforation plates as: simple,
scalariform, reticulate and foraminate.
5.
Characterize wood parenchyma.
6.
State
origins and functions of vertical and ray parenchyma.
7.
Define
tyloses and explain the ontogeny of tyloses.
Photos for this
study:
8-1:
Pinus
(pine): tracheid, macerated xylem
8-2: Pinus (pine): tracheid, macerated xylem,
simple and bordered pits
8-3:
Liriodendron
(tulip tree): macerated xylem
8-4:
Convolvulus
(hedge bindweed): l.s. stem HP
8-5:
Convolvulus
(hedge bindweed): l.s. stem
HP
TRACHEIDS
PHOTO STUDY 8-1 Pinus (pine): tracheids, macerated xylem
Like non-septate fibers, these tracheids are single cells. They differ from fibers in having only moderately thick walls and numerous large pits. Identify the large bordered pits. If bordered pits are in a wall facing you, there will be none in the sidewalls. But if a tracheid is turned so that the pits are in the sidewalls, there will be none in the wall facing you. This will become apparent in a sectioned stem in another photo study. In a bordered pit, distinguish between the pit aperture and the border.
PHOTO STUDY 8-2 Pinus (pine): tracheids, macerated xylem, simple and bordered pits
In this photo, distinguish between bordered pits (where a tracheid borders another tracheid), and simple pits (where a tracheid borders xylem ray parenchyma). VESSEL ELEMENTS
PHOTO STUDY 8-3 Liriodendron (tulip tree): macerated xylem
Observe an isolated vessel element (upper left). What kind of pits are there in the side walls? Are pits in any regular arrangement? Are the ends perpendicular to the long axis, or are they oblique? Have end walls been completely digested away, or only partially so? Partial digestion of an end wall of this sort is called a perforation plate; and when parallel bars of wall material are left between the open spaces, the plate is said to be scalariform. Very large pits with narrow borders may be seen in the vessel element; these were developed in a part of the wall that bordered a parenchyma cell. How do vessel elements compare with tracheids in length? Pit type? Wall thickness? A long column of xylem cells constitute a vessel if their end walls have been digested out, thus forming a long, hollow tube. Each cell of such a tube is called a vessel element, and in macerated xylem these elements become separated. PHOTO STUDY 8-4 Convolvulus (hedge bindweed): l.s. stem HP
Find a vessel element with ring-like (annular) thickenings in it. Imagine a short glass tube with rings inside it, the latter glued to the sides of the tube so that the plane of each ring is perpendicular, or nearly so, to the long axis of the tube. In appearance, that would correspond to the element you are looking for. The wall of such an element, corresponding to the glass of the tube is thin; it is the primary wall. The rings are secondary wall thickenings laid against the inner surface of the primary wall. Identify both. See that there are no end walls to the element.
PHOTO STUDY 8-5 Convolvulus (hedge bindweed): l.s. stem HP
This photo shows a vascular bundle that includes all of the xylem elements you saw in macerated tissue. Now you can see xylem elements in their natural positions in the stem. Identify annular, spiral, scalariform and pitted vessels, and also pitted tracheids and fibers. See vessel elements now as they constitute whole vessels. Note the end walls of two vessel elements has been almost completely digested. Which kind of element is nearest the pith? Which is farthest from the pith? (Pith is upper left in photo). If the rings of an annular vessel are far apart, note how the thin sidewalls sag inwards between the rings, due to growth pressure of neighboring living cells. Rings are frequently tilted to oblique positions, due to such pressure. Were it not for the rings, such vessels could be completely collapsed, thus stopping all water transport through them. Would you expect the same to be true of spiral vessels?
PHOTO STUDY 8-1 Pinus (pine): tracheids, macerated xylem
Like non-septate fibers, these tracheids are single cells. They differ from fibers in having only moderately thick walls and numerous large pits. Identify the large bordered pits. If bordered pits are in a wall facing you, there will be none in the sidewalls. But if a tracheid is turned so that the pits are in the sidewalls, there will be none in the wall facing you. This will become apparent in a sectioned stem in another photo study. In a bordered pit, distinguish between the pit aperture and the border.
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