Describe the internal structure of a sunflower stem.

(N/A) Sunflower is an annual plant with a herbaceous stem.
$A$ transverse section of a stained young sunflower stem,when observed under a microscope,reveals the following structures: $(1)$ Epidermis,$(2)$ Cortex,$(3)$ Stele.
$(1)$ Epidermis: It is the outermost protective layer,consisting of a single layer of parenchymatous cells. The outer walls of these cells are covered with a thin layer of cuticle. Multicellular stem hairs (trichomes) arise from these cells. Stomata are present between the cells,which facilitate gaseous exchange.
$(2)$ Cortex: The cortex is divided into three regions: Hypodermis,Main Cortex,and Endodermis.
$(i)$ Hypodermis: The region just below the epidermis is the hypodermis. It consists of $3-4$ layers of collenchyma tissue. The cell walls are thickened with cellulose and pectin. Intercellular spaces are absent. This tissue provides mechanical support.
(ii) Main Cortex: Located below the hypodermis,this region is composed of parenchyma tissue with abundant intercellular spaces. The cell walls are thin and made of cellulose. This region contains resin ducts. The cells store water and food.
(iii) Endodermis: The innermost layer of the cortex is the endodermis,which is single-layered. The cells are barrel-shaped. Since these cells contain starch grains,this layer is also known as the starch sheath.
$(3)$ Stele: The stele includes the pericycle,vascular bundles,and pith.
$(i)$ Pericycle: Located internal to the endodermis,it is multilayered and consists of alternating patches of sclerenchyma and parenchyma. The sclerenchyma patches located above the vascular bundles are called bundle caps.
(ii) Vascular Bundles: In a sunflower stem,$20-25$ vascular bundles are arranged in a ring. Each bundle is conjoint,collateral,and open. The xylem development is endarch. The vascular bundle consists of phloem,cambium,and xylem. The cambium present between the phloem and xylem is called fascicular cambium,which allows for secondary growth.