Vascular rays constitute 5-10% of the wood. These ingrowths are called tyloses Fig. Cork cambium is also known as Phellogen or Extra stelar cambium. Distinct annual rings are not formed in tropical plants. Structural details of Dracaena protective tissues: transverse sections from the epon- a and wax-embedded b-d materials. Old view: All the tissues situated outside the cork cambium is called bark. Anatomy of the vegetative organs of the pineapple Ananas comosus L.
The other cells of the secondary tissue are comparable in length with their initials. Other experiments carried out with this species revealed that the concentration of auxin in horizontal stems was up to seven times greater in the lower side of the stem axis than in the upper one Fisher et al. Hence, the annual rings are formed year after year. Both of them conduct water and food' in radial direction. In the direction of the centre, the bundles are relatively large in size and loosely arranged. Annual rings are not distinct in tropical areas which do not have long dry periods.
The latter is important in the boreal forests and in temperate plantations for the pulp and paper production. Other circumstances may cause the cambium to stop and start growth—forest fires, volcanic eruptions, defoliating caterpillar outbreaks, or extreme drought, for example—with false annual rings the result. In , this process produces , and shapes the plant into a with a thickened trunk. Anomalous Secondary Thickening in Dracaena Anomalous Secondary Thickening in Monocots Before going to the secondary thickening in Dracaena, first see what anomalous secondary thickening is and how the process of anomalous secondary thickening happens in plants. Ø Pith is the exact central portion of the stem. Ø In this mass of cells, the centrally placed cell metamorphoses into cells. The study of determination of age of the plant by these techniques is called Dendrochronology.
Therefore, the general designation of the secondary protective tissue, i. Ø Chlorenchymatous cells in the outer cortex can do photosynthesis. Functions of secondary meristem 1. Cambridge: Cambridge University Press; 1925. They take part in conduction of water and storage of food.
A few long-lived leaves also have secondary growth. Vascular Rays: The vascular rays or secondary medullary rays are rows of radially arranged cells which are formed in the secondary vascular tissues. Ø Parenchyma may be loosely arranged with many intercellular spaces. The heartwood becomes resistant to the attacks of bacteria and fungi due to the presence of antiseptic oils. These substances are collectively called extractives.
Ø The components of cortex and stele are together known as Ground Tissue. As a result of growing of secondary xylem, the diameter of heart wood increases Physiologically active wood is sapwood. The inner derivatives of the monocot cambium differentiate into cells of secondary conjunctive tissue and vascular bundles. In a much older stem the secondary xylem wood shows two regions namely duramen and alburnum. Ø Later divisions are irregular and to form a mass of cells. Phloem contains companion cells, sieve tubes, and phloem parenchyma.
The rays of the hardwoods usually are multiseriate many cells in width and hundreds of cells deep. In this way a complete ring of vascular cambium is formed. Ring bark is also formed in eucaiyptus. Secondary tissues : Tissues generated from the growth of a cambium. Conclusion The two meristematic tissues responsible for the secondary growth in the monocot species differ from the vascular cambium and the cork cambium of conifers and dicotyledons.
It provides us with a face-view of the sheet of vascular cambium. The second ring of vascular bundles is alternating in position with the first ring. Epidermis Epidermis is protective in function and creates the outermost layer of the stem. These are called secondary cortex or Phelloderm. As in the stems studied earlier, the ground tissue inside the vascular tissue is called the pith and that outside the cortex.
Ø Limited number of vascular bundles, usually 6 to 8 Ø Vascular bundles are arranged as a broken ring Ø Vascular bundles, conjoint, open, collateral or bicollateral. Ø The shape of these cambial cells varies. Heart wood is commercial source of Cutch Acacia catechu , Haematoxylin Haematoxylon campechianum , Brasilin Caesalpinia sappan and Santalin Pterocarpus santalinus. The parenchyma and bundle caps the cells among them comprise the pericycle in the stem of sunflower. This is called autumn wood or late wood. Über die verkorkten Abschlußgewebe der Monokotylen.