Question

Explain the internal structure of Cycas rachis.

Answer 1

1. The outermost layer is the epidermis and is covered by a thick cuticle.
2. The hypodermis is made up of two layers of sclerenchyma on the adaxial side and many-layered on the abaxial side.
3. The ground tissue is parenchymatous. The peculiar feature of the rachis is the arrangement of the vascular bundle i.e., in an inverted omega shape pattern.
4. Each vascular bundle is covered by a single-layered sclerenchymatous bundle sheath.
5. Vascular bundles are collateral, endarch and open.
6. A single-layered endodermis and a few-layered pericycle surround the bundle.
7. A diploxylic condition is present in the vascular bundles. (presence of both centripetal and centrifugal xylem).

• T.S. of Leaflet:

1. The leaflet of Cycas in the transverse section shows the presence of the upper and lower epidermis. The epidermal cells are thick-walled and are covered with thick cuticles.
   
2. The lower epidermis is not continuous and is interrupted by sunken stomata. The hypodermis consists of sclerenchyma cells to prevent transpiration.
   
3. The mesophyll is differentiated into palisade and spongy parenchyma. The cells of this layer are involved in photosynthesis.
   
4. The spongy parenchyma present in close proximity to the lower epidermis bears large intercellular spaces which help in gaseous exchange.
   
5. Layers of colourless, elongated cells which run parallel to the leaf surface from the midrib to the margin of the leaflet are seen.
   
6. These constitute the Transfusion tissue that helps in the lateral conduction of water. The vascular bundle has xylene facing the upper epidermis and phloem facing the lower epidermis.
   
7. The protoxylem occupies the centre, hence the bundle is mesarch. The vascular bundle has a sclerenchymatous bundle sheath.