Supporting tissues of the woody dicotyledonous stem are wood and bard. They are produced from the activity of two types of meristematic tissues; vascular cambium and cork cambium respectively.
As a result of this secondary growth (chapter 2) following secondary tissues which increase the thickness of stem, are produced.
- The medullary rays between the vascular bundles divide, forming a continuous cylinder of vascular cambium.
- The vascular cambium produces secondary xylem or wood towards the inside of the stem and secondary phloem towards the outside.
- Expansion of the vascular tissues stitches and breaks the epidermis and cortex, which are ultimately lost. The dividing cells, derived from the phloem, form a cork cambium.
Fig. 14.4 the diagrammatic cross section of three years old woody dicotyledonous stem shows the internal arrangement of the supporting tissues.
The cross section of woody dicotyledonous stem (fig. 14.4) shows the following internal arrangement of tissues.
1. Bark
The cork cambium, a special layer of cells in the outer region, produces layers of cells with thickened water proof walls called cork. All the cells outside the layers of cork eventually die as it cuts off all food supplies to them. Cork together with the dead epidermis and cortex collectively form bark of the tree. This is quite a distinct outer region that provides support to the woody dicotyledonous stem. Cork and cork cambium form an outer bark and cortex and phloem form an inner bark. Cork cambium also produces spongy tissues –lenticels in the bark through which exchange of gases takes place [fig14.5 (i) (iii)].
Fig/ 14.5 (i) cork cells (ii) bark of a tree with lenticels (iii) spongy tissues of lenticels
The rate of secondary growth in a stem varies with seasonal changes. This results in the formation of growth rings or annual rings, which may have a;
- Autumn wood as the xylem cells formed during autumn and winter i.e. during the period of slow growth, are small and thick walled.
- Spring wood as the xylem cells formed during spring and summer are large and have thinner walls during the period of fast growth.
“In temperate countries, one growth ting in a stem shows both types of xylem tissue formed during one whole year. This is known as an annual ring and the age of the tree can be determined by counting the number of rings present/ in tropical countries, growth rings are formed when favorable seasons ultimate regularly with unfavorable seasons”.
Practical work: microscopic examination of a young dicotyledonous stem and its comparison with woody dicotyledonous stem.
- Prepared slides of cross section of young and woody stem: sun flower.
- Observe with low power of a compound microscope, the prepared slides of cross sections of young (A) and woody sun flower (B) stems.
- Compare your slides with the fig. 14.1 and fig. 14.4 respectively to become familiar with the general regions of both of the stems.
- Draw a circle about 15 cm. in diameter with compass. Outline the vascular bundles and other regions as they appear in your slide A. Do not draw in any cell. Color different regions as seen in above figures.
- With high power of microscope, identify, and compare the structures and positions of supporting tissues of slide A and those of slide B.
- Answer and write down the following questions in your note book.
Q. (i) enlist the main supporting tissues of young and woody dicot stem.
Q. (ii) is the meristematic tissue present in both stems. If yes, name and write down their distinguishing features.
Q. (iii) Match the terms written under column A with those written under column B.
Column A Column B
Annual ring Turgidity
Heart wood Spongy tissue
Lenticel Seasons
Parenchyma Hard appearance
3.cambium
This is a small ring of meristematic tissues between bark and wood, which is responsible for increase in thickness. During the spring and summer, cambium is active in producing new cells by division. It forms new phloem tissues on its outside surface and new wood tissues on its inside surface. During one season of cambium activity, many more wood cells than phloem cells are formed. That is why woody area of a tree is always much greater in thickness than that of its bark.
Fig. 14.6 cross section of woody stems showing (i) annual rings; woody tissues (ii) winter and autumn wood
Medullary Rays
Medullary rays or pith rays appear as lines radiating from the center to the outside of the wood like the spokes of a wheel and help in horizontal transport of food materials.
Pith
This is a central core which is later replaced by heart wood. It never increases in size. Regardless of the size to which a tree may grow, its pith never increases beyond the amount present during the first year of growth.