leaf: growth leaf: external leaf: internal leaf: modified

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Leaves / introduction and growth

 

I. Leaves are the most active and conspicuous organs of plants.

The most important of their functions is absorbing sunlight to be used for photosynthesis. In order to do this, they expose large amounts of surface area to the environment. On a global basis, leaves produce more than 200 billion tons of sugars per year. Those sugars sustain most life on this planet.

Gas Exchange & conduction--; gas exchange is critical to the process of photosynthesis mentioned above. However as gas exchange occurs to obtain CO2, water is released which causes water to move up the xylem carrying other minerals with it in a primarily passive mode and allows the leaf to cool down. As we will study later, water moving through the xylem moves laterally to the phloem to aide in phloem transport of sap.

Support:: modified leaves called tendrils may allow viny like plants to climb up to increase light absorption

Absorption of nutrient ( mineral & water): the obvious example is carnivorous plants which capture insects, frogs or even small birds to increase their holdings of nitrogen. These plants live in nutrient deficient habitats ( i.e.. acidic,boggy areas where cation-ion exchange is not likely)and will not produce these leaves when fertilized.

As I mentioned in class, plants in very dry areas capture water and even minerals from fog or mist coming from nearby oceans. In some cases the leaves absorb the moisture directly through the leaves, in other cases they structurally funnel the condensed moisture to the stem and down to the roots.

Drugs and trichomes for herbivore defense: we will spend more time later in the semester on secondary compounds.. for now, acknowledge that leaves tend to hold high concentrations of these compounds for obvious reasons.

II. How leaves form:

Leaves are the most diverse of all plant organs-

  • they can be tubular, feathery, needlelike, cupped, smooth, sticky fragrant, or waxy.
  • they range in size from pinhead-sized leaves to 20-meter across
  • they range in number from millions to only one or two in the whole lifetime of the plant.

Leaves are formed by the coordinated efforts of several meristems, each of which is named for its position.

 

The earliest stage of leaf development is a small bulge at the shoot apex called a leaf buttress or primordium, which consists of 100 to 300 cells. It is formed by cellular divisions one to three cell layers below the overlying protoderm.

More cellular divisions and expansions produce a radially symmetrical cone called an apical peg, which has an apical meristem and a procambial strand that forms the leaf's midrib. The adaxial surface of the apical peg elongates slower than the abaxial surface, and arches the leaf primordium over the shoot apical meristem.

The leaf then forms an adaxial meristem that thickens the leaf. It forms an upper leaf zone and a lower leaf zone. The upper zone contains marginal meristems that form the flattened blade and stalklike petiole that attaches the leaf to the stem. The lower leaf zone forms the leaf base.

Continued growth of a leaf involves cellular expansion and division. These divisions continue until the leaf is one-half to three-fourths grown.

Cellular expansion forms most of the intercellular spaces in a leaf. Stomata differentiate soon after intercellular spaces form. Except for vascular tissues which differentiates from the base into the tip of the leaf, other tissues in leaves differentiate from the tip toward the base. Until it is 30%-40% of its final size, a growing leaf depends on the rest of the plant for its nutrition.