Discussion
The leaf outline was ā€˜V’ shaped in all Habenaria species. In all species studied, the cuticle was smooth on leaf surface. The cuticle thickness was greatest in all studied species. The cuticle is a thick, hydrophobic membrane composed of a polymer matrix (cutin) and associated with solvent soluble lipids (cuticular waxes) ( Rasmussen, 1987); Angelaet. al 2015). Plant cuticle plays an important role in the interaction of plant with environment, helps to reduce water transpiration ( Fahn 1982; Rindyastuiti et .al 2018).Habenaria species has a variation in cuticle thickness, the thick cuticle is an ecological adaption to reduce transpiration (Moreiraet. al 2013).
Epidermis is the outer cell layer of a leaf, act as a barrier between the leaves and environment it is also important in maintaining the external stimulus (Dietz and Hartung 1996). Epidermis is important to protect mesophyll and vascular bundles from higher solar radiation. Thick epidermis cell helps to reduce water transpiration.
Large epidermal cell in many species of orchids serve as water storage (Guan et .al 2011). Size of the epidermal cell is significantly different among species. Larger epidermal cells support the species to be more adaptive to the warmer environment . epidermal cell has various shape among species such as polygonal, isodiametric rectangular and elongated. (Aybake et.al .2010).
The hypodermis is a structure beneath the epidermis this structure can be present or absent in orchid species (Pridgeon 1982; stern 1997; Rindyastuti et.al 2018). Along with epidermis, hypodermis protect the mesophyll cells & vascular bundles. In the present study hypodermis was absent or not clear in the Habenaria species.
The mesophyll is important structure for leaves that is filled with photosynthesis component (chlorophyll) to assimilate nutrients. The number of mesophyll layers in the present study varies. The thicker leaves supports the leaves succulence. The succulence level of leaves related to the parenchymal capacity of mesophylls, to provide water supply for photosynthetic process and leaves cells, turgor mainly in the dry environment (Hsiao 1973; Lack and Evans 2001; Metusala et.al2017). In the present study Habenaria elwessi Hook.f has more mesophyll layers than the other Habenaria species. In present study Habenaria species shows mesophyll layer is made with homogenous parenchymatic cells.
The vascular bundle is a transport system containing xylem and phloem that are important in the water and nutrient transport (Lack and Evans 2001; Fahn 1982).
The vascular bundles arrangement in the mesophyll of Habenariaspecies has aligned in rows in the center of mesophyll. Vascular bundles arrangement in a single row was reported in Habenaria cornuta ,H . holothrix, H . monorrhiza, H. occidentalis, H. odontoptala, H. snowtenii, H. vaginatum. (Stern 1997) in present study sclerenchyma cells were not associated with xylem and phloem. Bundle sheath consisted of cells surrounding the vascular bundle. Bundle sheath cells were made with thin walled parenchymatic cells. Fibre bundles, spiral thickening, raphide bundles were absent or not clearly visible.