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Somatic embryogenesis is an ideal technique for the micropropagation of date palm using different explant tissue; however, histological studies describing the ontogenesis of plant regeneration are limited. This chapter provides a simple protocol for the histological analysis of the successive developmental stages of direct somatic embryogenesis induced from in vitro leaf explants. Direct somatic embryos are obtained from Murashige and Skoog (MS) medium containing 2 mg/L 6-benzylaminopurine. In order to observe the different developmental stages, histological analysis is carried out on samples at 15-day intervals for 60 days. Samples are fixed in formalin acetic alcohol and embedded in paraffin wax. Stain serial transverse and longitudinal sections, 8 μm thick, are stained with safranin-Fast Green. After 15 days on the induction medium, somatic embryos exhibit multicellular origin directly from the procambium cells, whereas the mesophyll and the epidermal cells are not involved in this process. After 2 months, several developmental stages (pre-globular, globular, early bipolar, bipolar, and cotyledonary-shaped) are observed. These embryos germinate after transferring to MS medium without plant growth regulators and rooting on 2 mg/L NAA-containing medium resulting in complete plantlets.

Two pot experiments were conducted on the 17 th and 11 th of march during 2015 and 2016 growth seasons respectively at the greenhouse, Dept. Agric. Botany, Faculty of Agriculture, Ain Shams University, Shoubra El-Kheima, Egypt to investigate the impact of drought stress on some growth, biochemical and anatomical parameters of Thymus vulgaris L. Plants were exposed to two different irrigation levels: 70-80% and 30-40% of water holding capacity (WHC) as well-irrigated and drought stressed plants respectively. The results indicated that all investigated growth parameters and leaf photosynthetic pigments were decreased significantly by exposing to drought stress. There were significant increase in the concentration of reactive oxygen species (ROS) as indicated by measuring of H2O2 and malondialdehyde (MDA). A similar trend was observed in respect to osmolytes including proline, total soluble sugars and free amino acids. Also, drought stress increased significantly total soluble phenols and the specific activity of Phenylalanine ammonia-lyase (PAL) and Polyphenol oxidase (PPO), whereas, no significant differences were detected in peroxidase (POD). The anatomical examination showed that there were several significant changes associated with water limited supply in the stem and leaf of plants.