The region from the pericycle to the pith in the center of the root and stem is called the stilt or intrastilt zone. Its various parts are-
(i) Pericycle: One or more layers of pericycle under endoderm. It is composed of parenchyma or scarenchyma or both tissues. Sclerenchyma tissue sits like a cap on top of the phloem. It is called Bundle cap. Pumpkin and sunflower stems have hard busts. Pericycles are present in the roots and stems of pteridophytes and in the roots of flowering plants. In the stems of pumpkin and kumarica plants, the pericycle is multi-layered and composed of scarenchyma tissue. It is absent in parasites and aquatic plants. In modern concepts, the pericycle is part of the phloem.
Function of pericycle: It produces apical roots in the stem. It gives strength to the plant. It stores food. It creates side roots.
(ii) Medulla : The part surrounded by vascular bundles located in the center of root and stem is called marrow. It is composed of parenchyma or sclerenchyma cells. The cells are spherical, thin-walled, and capsuled. It originates from the ground tissue. The medulla is absent in the roots of dicotyledonous plants. Many times the medulla is destroyed in the center of the root and stem and creates voids. It is called lysigenous cavity. Stems of pumpkin, papaya, bamboo, cucumber etc. contain lysigenous.
Function of medulla: It stores food. Sclerenchymaske¬renchyma gives the plant strength.
(iii) Medullary ray: The part from medulla to pericycle through the vascular bundle is called medullary ray. It is composed of narrow and elongated parenchyma cells. The medulla is found in the stems of dicotyledonous plants.
Function of marrow: It stores water and food. Transports water and food. It forms secondary tissue.