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Photo Unit 19 >




  1. List stages in leaf development as: a) initiation; b) early differentiation; c) origin of lamina; e) histogenesis of laminar tissues.
  2. Describe the origin of leaves in Allium (onion).
  3. State the layer or layers that give rise to the leaf, comparing the situation in gymnosperms and angiosperms both dicots and monocots.
  4. Relate laminar development with prolonged marginal meristem activity.
  5. Sequence events of meristem activity and restriction of meristem activity during leaf development.
  6. Interpret internal leaf tissue histogenesis given a scheme of tissue differentiation and a sectional diagram of young leaf primordium.
  7. Sequence ontogeny of the leaf development as a) midrib; b) lamina; c) petiole; d) stipules when present.
  8. List factors influencing leaf shape, especially simple vs compound leaves.


  10. Discuss the development of a multiseriate epidermis and characterize the structures found therein.
  11. Characterize the vascular system in large veins to their endings in vein islets especially with regard to size and contents of vascular bundles.
  12. Name two explanations for the origin of blind veinlets in areoli and analyze, distinguishing monocot from dicot leaves on the basis of these veinlets.
  13. Describe the bundle sheath in structure and function when: a) it surrounds large veins, and b) small veins both in dicots-monocots and in gymnosperms.
  14. Differentiate palisade and spongy tissue based on these criteria:  a) relative space/tissue ratio; b) cell shape; c) concentration of chloroplasts; d) presence in monocots-dicots gymnosperms; e) origin.
  15. Discuss inverted bundles in leaves such as Iris.
  16. Define adaxial, bifacial and dorsiventral as it applies to leaves.

Photos for this study:

Early Ontogeny

18-1: Salix (willow); l.s. stem tip

18-2: Salix (willow); l.s. stem apex with older leaf primordia

18-3: Salix (willow); T.s. stem tip. Young primordium

18-4: Salix (willow); T.s. stem tip Older primordium

18-5: Salix (willow); T.s. stem tip. Older primordium

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.

Mature Dicot Leaf

18-1A: Euonymous T.s. leaf lamina (blade)

18-2A: Syringa T.s. leaf midvein HP

18-3A: Rhododendron (rhododendron); Paradermal section palisade/spongy.

18-4A: Rhododendron (rhododendron); Paradermal section spongy


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.

Mature Monocot Leaf

18-1B: Tulipa (tulip); T.s. leaf

18-2B: Iris (iris); T.s. “I-leaf”

18-3B: Iris (iris); T.s. “V-leaf” (ensheathing base)


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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