Tag: internal structure of root, stem and leaf

More distinct annual rings are formed in that regions, where climatic variations are sharp. More distinct annual rings are formed in temperate plants as the climatic conditions of temperate regions are not uniform.

The cells of the seive tube elements contains protoplasm. a large vacuole however nucleus is lost at maturity. The conduction of food material takes between the cells through seive plants. The flow of food (sucrose), its loading etc is controlled by companion cells. Each seive tube cell is associated with one companion cell.

Hypodermis is a distinctive layer of mechanical tissue present in some dicot and monocot stems. It is collenchymatous and present as 3-4 layers. It provides mechanical support and elasticity, also stores food.

Cortex is the outermost layer of the stem or root of the plant. It is surrounded by a parenchymatous layer. The cortex is unspecialized cells which help as conducting tissue in the plants.

Peripheral vascular bundles are generally smaller than the centrally located ones because the phloem parenchyma is absent and the water containing cavities are present in the vascular bundles which are both in monocot and dicot stem.

Most of the features of the dicot and monocot roots are similar except the vascular bundles. In the dicot root, there are 2-6 bundles of xylem and phloem alternating whereas in monocots the number of xylem and phloem strands are more than 8. Further, the xylem vessels in dicots are polygonal and those of monocots are oval.

• In dicot stem, the secondary vascular tissues originate as a result of the cambial activity. 
• The cambial cells divide tangentially again and again and produce secondary tissues. 
• Some of the cells of single-layered pericycle become meristematic, which divides and form a few layers of cells, phellogen.
• Hence, In a dicot stem, pericycle gives rise to Phellogen.
• So, the correct answer is 'Phellogen'.

The earliest vascular plants had stems with a central core of vascular tissue. This consisted of a cylindrical strand of xylem, surrounded by a region of phloem. Around the vascular tissue, there might have been an endodermis that regulated the flow of water into and out of the vascular system. Such, an arrangement is termed a protostele, which is the simplest type of stele. Actinostele is a variation of the protostele in which the core is lobed or fluted. This stele is found in many species of club moss. Actinosteles are typically exarch  and consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem. Exarch protosteles are a defining characteristic of the lycophyte lineage.

Knots are common blemishes in trees, often causing lumps or holes within the trunk of the tree itself. In most cases knots are caused by the natural growth of the tree, though the specific circumstances under which they form determines how they will appear. As a tree grows and increases the circumference of its trunk, the growing trunk begins to overtake the branches that grow out from it. Knots form around these branches, building up trunk material as the tree continues to expand. Since, the branches are still growing as they are overtaken by the trunk, the knot that forms is solid and contains living wood throughout. The wood of the knot is typically tougher than the surrounding wood and may form a bulge around the branch emerging from its center. So, knots are formed in wood due to bases of the branches get buried in main stem and not because leaf scars, insect bites and injuries. 
Therefore, the correct answer is option B.

Phellem is outermost layer of cork cambium, which is suberised. Hence, this is the first layer to be found. 
Endodermis is one the layer of cortex, which is to be found under the phellem.
Pericycle is the layer present between endodermis and vascular bundles. 
Dicotyledonous stem possess conjoint, collateral vascular bundle, where xylem is internal and phloem is external. Therefore, phloem is to be encountered after the pericycle. Hence, option C is correct.