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The aim of this work was to determine the anatomical adaptations of leaves and stems of four species of Poaceae (Cynodon dactylon L. Pers., Chloris barbata SW. (Chloridoideae), Setaria verticillata L. P. Beauv., and Panicum coloratum L. (Panicoideae)) growing naturally at Al-Ha'ir region near Riyadh, Saudi Arabia. Cross-sections of the leaves revealed that the epidermis cells were spherical to oval and formed one layer with a thick cuticle as well as numerous bulliform cells in a fan shape and prickles. Ground tissue (mesophyll) consists mostly of chlorenchyma cells. A large vascular bundle surrounded by two bundle sheaths, outer sheath parenchyma and inner sheath sclerenchyma were observed in C. barbata and C. dactylon, while one bundle sheath of sclerenchyma surrounds the vascular bundle in S. verticillata and P. coloratum, while one bundle sheath with chlorenchymatous cells surrounded the small vascular bundles. Our results showed that all species contained a Kranz anatomy indicative of a C4 photosynthetic pathway despite belonging to two different subfamilies, Panicoideae and Chloridoideae. Oil droplets appeared in the mesophyll tissue of P. coloratum and S. verticillata. Cross-sections of stems revealed an epidermis which consists of one layer of cells with spherical to oval shape and had a thick cuticle. Ground tissue contains strands of chlorenchyma cells followed by sclerenchyma tissue surrounding vascular bundles, thereby making a continuous cylinder. The vascular bundles were scattered in the ground tissue, with each vascular bundle surrounded by a single sclerenchymatous bundle sheath. Our results indicate that these plants were characterized by anatomical adaptations that enhance drought-tolerance capabilities, facilitating survival in arid and semi-arid regions such as Al-Ha'ir and thus these plants can be used to increase vegetation cover and pasture area in dry environments. Key words: Anatomy, adaptations, Poaceae, Cynodon dactylon, Chloris barbata, Setaria verticillata, Panicum coloratum.

The pollen morphological characters of 6 species belonging to 4 genera of the subfamily Stachyoideae (Lamiaceae) growing naturally in Saudi Arabia were investigated with the aid of light microscope (LM) and scanning electron microscope (SEM), to find new features that might increase knowledge of pollen morphology of the species, and also to help the taxonomic characterization of the Stachyoideae genera. The morphological characters studied were size, shape, tectum surface ornamentation, number and type of the colpi. The study indicated that the average size of the pollen was different among the species since the smallest size was that of Nepeta deflersiana (P = 25.2 ± 2 and E = 15 ± 3) while the largest size was that of Salvia aegyptiaca (P =36.5 ± 2 and E = 30.3 ± 2). Pollen shape is spheroidal to sub-spheroidal or prolate. The fine structure of the exine of pollen was slightly different among investigated species. The number and type of colpi of the pollen in species studied were 6-zonocolpate type except those of N. deflersiana and Otostegia fruticosa ssp. schimperi having 3-zonocolpate. Key words: Pollen grains, pollen morphology, Lamiaceae, Stachyoideae, Saudi Arabia.

Pollen micromorphological characters such as pollen size, shape, number of associated monads, colpi and ornamentation of the tectum surface, of seven species of Acacia belonging to the family Mimosaceae were studied, using light microscope (LM) and scanning electron microscope (SEM). The study showed that the pollen size range from 42.1 to 69.4 µm, pollen shape is round to semi- round, the number of associated monads is 16 or 32, and the colpus is Y - H shaped. The tectum surface ornamentation among the species is variable, it is foveolate in Acacia ehrenbergiana; psilate-foveolate in Acacia nilotica, Acacia laeta and Acacia negrii; and micro-reticulate in Acacia farnesiana, Acacia oerfota and Acacia tortilis ssp. raddiana. An artificial key based on micromorphological characters is provided. Key words: Mimosaceae, Acacia, pollen morphological characters, Saudi Arabia.

Investigation was carried out to find whether enhanced ultraviolet radiation influences the Malva parviflora L., Plantago major L., Rumex vesicarius L. and Sismbrium erysimoids Desf. of some annual desert plants. The seeds were grown in plastic pots equally filled with a pre-sieved normal sandy soil for 1 month. The planted pots from each species were randomly divided into equal groups (three groups). Plants of the first group exposed to white-light tubes (400-700 nm) 60 w and UV (365 nm) 8 w tubes. The second group was exposed to white-light tubes (400-700 nm) 60 w and UV (302 nm) 8 w tubes. The third group was exposed to white-light tubes (400-700 nm) 60 w and UV (254 nm) 8 w tubes, respectively, for six days. The results indicated that the chlorophyll contents were affected by enhanced UV radiation. The chlorophyll a, b, and total contents were decreased compared with the control values and reduced with the enhanced UV radiation, but the carotenoid was increased compared with the control and also reduced with the enhanced UV radiation. So, the contents of chlorophylls varied considerably. M. parviflora showed the highest constitutive levels of accumulated chlorophyll a, b, and total chlorophyll (0.463, 0.307 and 0.774 mg g(-1) f w) among the investigated plant species. P. major showed the lowest constitutive levels of the chloroplast pigments, 0.0036, 0.0038 and 0.0075 mg g(-1) f w for chlorophyll a, b, and total chlorophyll at UV-365 nm, respectively. The protein content was decreased significantly in both root and shoot systems compared with the control values but, it was increased with increasing wave lengths of UV-radiation of all tested plants. R. vesicarius showed the highest protein contents among the investigated plants; its content was 3.8 mg g(-1) f w at UV-365 nm in shoot system. On the other hand, decreasing ultraviolet wave length induced a highly significant increase in the level of proline in both root and shoot of all tested plants. From the results obtained, it is suggested that proline can protect cells against damage induced by ultraviolet radiation. Statistically, the variations of the studied metabolic activities were significant due to UV radiation treatment in shoot and root system of all investigated plant species.