Describe the internal structure of the leaflet of cycas and list out the xerophytic adaptations seen in it.
Hint: The Cycas plant grows to a height of 8-15 feet and resembles a small palm. Its roots are of two types: normal roots and coralloid roots. The leaves of the Cycas are also of two kinds—foliage leaves and scaly leaves. Each leaf consists of a petiole, rachis, and lateral pinnae. The young leaves exhibit circinate vernation.
Complete answer:
The internal structure of a Cycas leaf can be distinguished by a swollen midrib and two lateral wings that curve downward. The outermost layer of the leaf is made up of a thick-walled epidermis covered by layers of cuticle. The upper epidermis forms a continuous layer, while the lower epidermis is interrupted by numerous sunken stomata.
Beneath the upper epidermis lies the sclerenchymatous hypodermis, which is thicker in the midrib area. The hypodermis is absent below the lower epidermis, except in the midrib region. The mesophyll is divided into palisade and spongy parenchyma, with the latter found only in the wings directly beneath the lower epidermis. These cells contain chloroplasts and are loosely arranged with intercellular spaces filled with air. Secondary transfusion tissues are present in the mesophyll cells. A single vascular bundle is found in the midrib section of the leaflet. These vascular bundles are conjoint, collateral, and open, with a diploxylic structure. The centrifugal xylem is triangular and well-developed, with endarch protoxylem. The phloem is arc-shaped and separated by cambium, consisting of sieve tubes and phloem parenchyma. Companion cells are absent. Parenchyma cells in the midrib contain calcium oxalate crystals.
Xerophytic adaptations of Cycas leaf:
> The thick cuticle helps prevent excessive water loss through transpiration.
> Sunken stomata are present on the leaves, reducing water loss.
> The hypodermis is 1-2 layers thick and contains lignified cells, along with mucilage canals that store water and minimize transpiration, which is characteristic of xerophytic plants.
Note:
There are approximately 200 species of Cycas. These plants are native to tropical and subtropical areas and can be found in regions such as Central America, South America, Africa, Asia, and Australia. They are also cultivated as decorative plants.