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LEAVES, conclusion...

Determinants of fall color.
Genes - species and variety differences.
Latitude and climate - color strongest at higher latitudes. Again, species, but also cool nights.
Seasonal fluctuations.
Again, cool nights, but also soil moisture, soil acidity, etc.
Drought a factor, as is too much rain.
At the same time this is going on, sugars and other substances in the leaf are recovered into the rest of the plant.
Much of the leaf is broken down, leaving just cellulose. \

<>What role does daylength and plant hormones play in leaf senescence?
What happens to all the proteins, sugars, minerals etc that are in the dying leaf?
What about the cell walls?

SECONDARY GROWTH

We’ve now talked about primary growth, the soft, unhardened, tender parts of the plant formed during extension of roots and shoots.

In many plants whose life cycle lasts just one growing season, that’s all there is.
The plant flowers and dies. They complete their life cycle in that one year, from germination to flowering. Your pet plant, Brassica, is an example.
It’s an ANNUAL.

Other plants are BIENNIALS.
They complete their life cycle in two years. During the first, they store reserves in underground roots, buds or stems. During the second year, they undergo another round of growth, then flower and die.

Many plants are PERENNIALS. They grow year after year. Some do this by dying back each dormant season to underground roots, stems and buds. E.g. bulbs like daffydowndillies.

But many other perennials are WOODY. They undergo secondary growth. They lose only part of their shoot, the leaves. The stems become woody, or hardened and protected. They go dormant in the winter or dry season, then resume growth later on.

How does this happen?

Let’s go back to the primary stems, and the cells left behind in open vascular bundles.
In the case of bundles, the cells start dividing again during the first year of growth.
fig. 6.5

They are linked to the same cells in other bundles by divisions in the parenchyma cells between bundles. This too creates a ring of dividing cells. This is the VASCULAR CAMBIUM. fig. 6.6

It produces secondary xylem inwards, secondary phloem outwards. At the end of the first growing season, vascular cambium ceases activity, leaving a ring of secondary xylem.

In the following spring, another layer of secondary xylem is produced.

These are called ANNUAL RINGS. FIG. 6.7
Also fig. 6.6 -- HOW OLD IS THIS STEM???

****Note that the xylem differs with the growing season. Early, when rain is plentiful, the water conducting elements are wider. As summer ensues and rain is scarcer, the xylem elements narrow. So, it’s easy to distinguish between the summer xylem and the following spring xylem, making the annual rings obvious.

Sometimes, if a wet period occurs after a dry spell, a new spurt of wide elements might form. Thus, two 'annual' rings in one season.

Of what use are annual rings to scientists?

What is WOOD, botanically?

What is heartwood and sapwood? See your book; Fig. 6.9

As circumference of stem increases during secondary growth, and as secondary phloem pushes outward,

Epidermis cracks and sloughs off.
Why is this a potential problem for the plant?

A new dermal layer forms as cortical cells resume division.
    Cork Cambium

    The products of these divisions specialize as cork cells, which protect the plant in place of the epidermis.
    Cork pattern on various trees is species specific. Allows us to identify trees even in winter.

What is PERIDERM?

As stem expands, old cork also peels off.
    Live cells in secondary phloem resume division, giving rise to more cork cells. What is meant by bark, in the botanical sense and the popular sense?
        What is inner bark? Outer bark?

More about the vascular cambium (Fig. 6.4)
What are initials? Fusiform initials?

HOW DO THE INITIALS PRODUCE XYLEM INWARDS AND PHLOEM OUTWARDS?

HOW DOES THE CIRCUMFERENCE OF THE VASCULAR CAMBIUM INCREASE

    What are periclinal divisions and what do they produce?
    Anticlinal divisions? What is their function?