Unbūb denotes a tube-like stem or slender shoot. Here the explanation of al-saqiyy points to a watered palm and its tender growth, while al-mudhallal adds the condition of being trained or bent for care.
The resemblance concerns slender length, smoothness, and tenderness. Substituting an unrelated modern object such as bamboo would lose the poem's particular agricultural and desert setting.
The date palm appeared in verse 35 for a crowded fruit cluster; here a different part of the plant supplies an image of a smooth slender shoot. Reusing a familiar natural source with a different selected feature is part of the poet's descriptive technique.