BIO 206 PLANT MORPHOLOGY

BIO 206

PLANT MORPHOLOGY

LECTURE NOTES 5th WEEK

DR. AYDAN ACAR ŞAHİN

Secondary growth

^• In many plants, vegetative development is completed after the maturation of primary tissues. However, in many herbaceous and woody dicotyledons, formations of new tissues continue even after the maturation of primary tissues.

• The production of these new tissues is attributable to the lateral meristem, which includes cork and vascular cambium.

These cambia produce new tissues for effective protection, conduction and mechanical strength – a phenomenon termed secondary growth.

• Secondary tissues are formed by the cambium, which is normally present in dicotyledonous roots and stems. Though some monocot stems possess nonvascular cambia, which produce secondary tissues, it is regarded as anomalous. So normal secondary growth occurs in dicots and gymnosperms only, and it causes increase in thickness both in intrastelar and extrastelar region of stems.

• Secondary tissues are produced both at fascicular and interfascicular region. Normally equal proportion of secondary xylem are differentiated in all segments of the cambium ring on the inner side while the secondary phloem are formed towards periphery in equal amount in all the segments of the cambium ring but they (secondary xylem and phloem) are not of equal quantity.

Generally, more amount of secondary xylem is differentiated in

contrast to secondary phloem. The cells of cambium ring also

divide radially to produce ray cells or medullary cells. The ray

parenchyma cells are arranged radially in secondary xylem and

phloem.

• The cross-section of the gymnosperm wood has even rings of the year seen with the naked eye. Seasonal differences have a large impact on plant life activities. Spring is the most active plant. The plant needs large amounts of water in spring. Therefore, the diameter of the tracheids occurring in the spring is wide and the walls are thin. The light colored ring formed by this type of tracheids in the wood is called spring wood.

• With the start of summer, the amount of water in the environment decreases and a decrease in the activity of the plant is seen. In this circuit, smaller diameter and thick- walled tracheids are formed from cambium. The dark ring they create is called summer wood. Wood formation ends in autumn. The large tracheid wood, formed in the following spring, appears light colored on dark colored and small diameter cells of autumn.

• Thus, a light colored ring followed by a dark ring forms a year ring. Self-arms of gymnosperm are quite common.

• Nutrients are stored in their own arms. This nutrient is often starch.

Transversal:

Longitidunal radially

There are two types of rays in gymnosperms; homocellular rays (when rays comprise of only

parenchyma) and heterocellular

rays (when it consists of both

parenchyma cells and tracheids).

Longitidunal tangentally

Transversal Longitidunal-radial Longitidunal-tangental