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PHOTO UNIT 18:
ONTOGENY OF THE LEAF
OBJECTIVES:
PHOTO STUDY 18-1 Salix (willow): l.s. stem apex.
Identify the tunica and corpus of the promeristem and, back from the apex, the three primary meristems. How many layers is the tunica? Observe leaves of different ages, the youngest, of course, on the side of the apex. At this stage of development, the young leaf resembles a finger in shape, having grown upward from its nodal origin, and having curved slightly over the stem apex. What you see of the leaf here represents only the petiole-midrib axis of the fully developed organ: there is no blade part this early in development. PHOTO STUDY 18-2 Salix (willow): l.s. stem apex with older leaf primordia.
See how close together the leaves are, thanks to the shortness of the internodes. Recognize lateral growing points in the axils of partly matured leaves. Opposite an occasional leaf, you may make out a leaf gap in the procambial stele. Leaf gaps won’t be visible, however, for all leaves in the section. How do you account for this difference? Once before you traced procambium strands upward to the stem tip, and discovered that none could be traced into the tip proper, but only into the young leaves. Do you find that to be true here also? PHOTO STUDY 18-3 Salix (willow): T.s. stem tip.
For this study, observe the leaf primordium that is almost circular, such as that of a finger would be, except for a slight flatness on the adaxial side. Identify protoderm, the small-celled procambium, and ground meristem. Does protoderm show evidence of maturing faster on one side than on another? Again, this is only the petiole-midrib axis of the leaf.
PHOTO STUDY 18-4 Salix (willow): T.s. stem tip.
Select a sectioned leaf in which right and left halves of the lamina (blade) have just begun to differentiate. Do these halves develop from the sides of the leaf axis halfway between top and bottom, or not? Which side is dorsal and which ventral? Again recognize the three primary meristems and note any evidence of maturing.
PHOTO STUDY 18-5 Salix (willow): T.s. leaf primordia
Select a leaf with lamina further developed, exhibiting protoderm externally and three distinct tissue regions internally. Identify the marginal initial in each half and determine what it has contributed to the expanding leaf. The marginal initial that you see here must be thought of as one member of a whole row of initials along one edge of the young blade, only one of which could be included in a cross section.
Identify sub-marginal initials also. From them, trace back into the lamina to see what contributions they have been making. Note the great thickness of the midrib portion of the young leaf. Discover differentiating procambium that is destined to become a vascular bundle.
PHOTO STUDY 18-6 Salix (willow): Transverse section of an older leaf.
See what changes have taken place in protoderm to yield what will become the mature epidermis. See that the central tissue is adding one layer of its cells to the palisade, thus making that tissue two cells deep. Most of the central tissue will become spongy tissue, but you will see that some of it (parts that differentiated into procambium) has matured into vascular bundles. Palisade and spongy tissues, will mature from ground meristem, and will constitute the mesophyll of the leaf. Note the modification of these tissues where they abut on vascular bundles, adding to the latter the bundle sheaths. Note the relative positions of xylem and phloem in a bundle.
PHOTO STUDY 18-1 Salix (willow): l.s. stem apex.
Identify the tunica and corpus of the promeristem and, back from the apex, the three primary meristems. How many layers is the tunica? Observe leaves of different ages, the youngest, of course, on the side of the apex. At this stage of development, the young leaf resembles a finger in shape, having grown upward from its nodal origin, and having curved slightly over the stem apex. What you see of the leaf here represents only the petiole-midrib axis of the fully developed organ: there is no blade part this early in development.
PHOTO STUDY 18-1A Euonymous: T.s. leaf lamina (blade)
How do epidermal cells on the adaxial side compare with those on the abaxial side? Also identify the very thin layer of cutin against the outer surface. Do you find stomata in both upper and lower epidermal layers? Note several to the left of the vein. Guard cells are small, but thick-walled. See that a conspicuous air space lies immediately beneath each stoma. This air space is continuous with other spaces in the spongy part of the mesophyll. The palisade mesophyll is made up of elongated cells, all arranged with their long axes perpendicular to the epidermis. How deep is this palisade? Are its upper and lower cells of equal length? Are there air spaces between the cells? Are cell walls thick, or thin? Turn your attention to the midrib region, which protrudes below the surface of the blade. The most conspicuous part of the midrib is the vascular bundle. PHOTO STUDY 18-2A Syringa, T.s. leaf midvein HP
Identify the cambium in the vascular bundle, and the secondary xylem and phloem that it has added. Primary and secondary xylem are hard to distinguish here, since the cells of both are in regular rows; but the former includes more parenchyma tissue than the latter does. See that rows of vascular elements are separated by uniseriate rays, the latter extending through the cambium into secondary phloem. In the phloem tissue recognize sieve tubes, companion cells, and parenchyma. PHOTO STUDY 18-3A Rhododendron (rhododendron) Paradermal section
Pay particular attention to the distribution of veins. See that the larger veins anastomose with one another, making a complicated network, the meshes of which are occupied by mesophyll tissue. Mesophyll tissue that fills a single mesh of this vascular network is called a vein islet. See that short branches from the boundary veins enter an islet and end freely there against mesophyll cells. Such branches are, of course, the vein endings of xylem only. See that veins are so close together that no mesophyll cells can be far removed from at least one vein. This mesophyll tissue contains the palisade (regular circular in cross section, and the irregular spongy mesophyll).
PHOTO STUDY 18-4A Rhododendron (rhododendron) Paradermal section
Note the irregular shape of mesophyll cells here, arm-like extensions of one cell meeting similar extensions (or lobes) of other cells so that all together constitute a net-like organization in three dimensions, with air occupying the large meshes of the net. This is spongy mesophyll.
PHOTO STUDY 18-1A Euonymous: T.s. leaf lamina (blade)
How do epidermal cells on the adaxial side compare with those on the abaxial side? Also identify the very thin layer of cutin against the outer surface. Do you find stomata in both upper and lower epidermal layers? Note several to the left of the vein. Guard cells are small, but thick-walled. See that a conspicuous air space lies immediately beneath each stoma. This air space is continuous with other spaces in the spongy part of the mesophyll. The palisade mesophyll is made up of elongated cells, all arranged with their long axes perpendicular to the epidermis. How deep is this palisade? Are its upper and lower cells of equal length? Are there air spaces between the cells? Are cell walls thick, or thin? Turn your attention to the midrib region, which protrudes below the surface of the blade. The most conspicuous part of the midrib is the vascular bundle.
PHOTO STUDY 18-1B Tulipa (tulip): T.s. leaf
In contrast with the leaves you have previously studied, see that this one has no differentiation of the mesophyll into palisade and spongy portions. This is commonly true of the monocotyledons, though there are exceptions. Note the shape of epidermal cells in this view. Compare the frequency of stomata on the two leaf surfaces. Note that each stoma lies opposite an internal air space. Are vascular bundles enclosed within bundle sheaths here or merely a cap? Small bundles reduced to mere xylem elements. No cambium will be found in any of the veins. Note the bundles are alternately closer to adaxial and abaxial surfaces. Note also that the fibrous cap remains toward the outer side of the bundle. This leaf is called an I-leaf, and is at the more distal end of the blade.
PHOTO STUDY 18-2B Iris (iris): T.s. “I-leaf”
If you were observing a single leaf base (no blade here), you would discover that the basal portion of each leaf (often half or more) is apparently folded lengthwise down the middle so that the two halves closely ensheath younger leaves of the same bud. It would form a V-shaped scar if removed. Toward the distal end, the two sheathing halves apparently fuse together. Now look at the section of the “I-leaf”. Especially significant here is the distribution and orientation of vascular bundles. Discover that the bundles are differently oriented: in one group the phloem tissue is directed toward one leaf surface, while the alternating group is directed toward the opposite surface. It would appear as though the leaf of Iris has two lower surfaces and no upper surface at all. PHOTO STUDY 18-3B Iris (iris): T. s.V-leaf (ensheathing base).
The section is sharply V-shaped, the outside surface of the V being the lower (dorsal) surface of the leaf base, and the inside surface being the upper (ventral) surface of the leaf base. Keep this relationship in mind as you study the orientation of vascular bundles. See that every bundle has its phloem portion directed toward the dorsal surface, whether in one arm of the V or in the other. This, of course, is the normal orientation in any leaf. If the two arms of the V should now approach each other and fuse together, it is obvious that there would be two groups of bundles, oppositely oriented. Such a phenomenon would account for the inverted bundles of the Iris leaf, which you saw in the preceding study, and would partially explain why this leaf cannot be said to have a true lamina or blade.
PHOTO STUDY 18-1B Tulipa (tulip): T.s. leaf
In contrast with the leaves you have previously studied, see that this one has no differentiation of the mesophyll into palisade and spongy portions. This is commonly true of the monocotyledons, though there are exceptions. Note the shape of epidermal cells in this view. Compare the frequency of stomata on the two leaf surfaces. Note that each stoma lies opposite an internal air space. Are vascular bundles enclosed within bundle sheaths here or merely a cap? Small bundles reduced to mere xylem elements. No cambium will be found in any of the veins. Note the bundles are alternately closer to adaxial and abaxial surfaces. Note also that the fibrous cap remains toward the outer side of the bundle. This leaf is called an I-leaf, and is at the more distal end of the blade.
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