Vessels and rays in the trunks of a fully grown and fruiting tree of Citrus sinensis were studied with the aid of light microscope. 50 measurements of each of the vessels and rays were carried out by using ocular micrometer to determine their lengths and diameters in micrometer (µm). The data obtained were subjected to ANOVA and Duncan multiple Range Test (D.M.R.T)to determine their average means. The average length and diameter (µm) of the vessels were 213.07±35.64 and 69.35±13. 87 respectively. On the other hand , the average height(µm) of rays was 313.65±62.02 while the diameter (µm) was averagely 44.87±23.30. From the axial direction of the trunk, the average lengths (µm) of the vessels at the base, middle and top regions were 213.38+37.72, 208.32±34.31 and 217.62±34.58 respectively, whereas, their average diameters (µm) were 76.34±10.90, 68.40+16.24 and 63.33±10.68 for the same directions and regions respectively. Similarly, the average lengths of vessels along the radial direction of the trunk were 209.97±32.03, 210.80±41.28 and 218.45±32.85 for the core, middle and outer-wood respectively whereas their average diameters were 65.55±13.42, 71.39±15.45 and 71.12±11.96 for the same direction and regions respectively. There were no significant differences (P≤ 0.05) between the average length of vessels in the base, middle and top regions as well as the vessels in the core, middle and outer-wood of the trunk respectively. However, significant differences (P≤0.05) existed between the average diameters of the vessels at the base, middle and top regions of the trunk as well as between the core, middle and outer-woods respectively. The average heights of the rays along the axial directions of the trunk did not differ significantly (P≤0.05) at the base, middle and throughout the top regions. Similarly, insignificant differences existed in the average length of rays in the core, middle and outer-wood of the trunk through the radial directions. Conversely, the diameter of the rays showed significant differences (P≤0.05) between the base, middle and top regions as well as between the core, middle and outer-wood of the trunk respectively. Structurally, transections through the trunk of this species revealed banded-multiseriate apotracheal parenchyma tissues, solitary vessels, doublet vessels elements, short and opened ended vessel elements as well as triplet vessels. The rays, on the other hand were heterogenous and properly distributed. Photomicrograghs of the anatomical features were prepared to further illustrate the type and nature of the vessels and rays observed in the trunk. It was proposed that the results obtained in the present study will be useful for researchers in the fields of Plant anatomy, Plant pathology, Plant taxonomy, Plant breeding, Plant ecology and Forestry. The results were further recommended for Pulp and Paper industries, wood construction industries and the like.
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