Explanation:

Phloem
Like xylem, phloem is also a complex tissue. It conducts food materials to various parts of the plant. The phloem elements which are formed from the procambium of apical meristem are called primary phloem. The phloem elements which are produced by the vascular cambium are called secondary phloem. The secondary phloem is situated Just outside the vascular cambium. The primary phloem elements that develop first from the procambium are smaller in size called the protophloem, whereas those develop later are larger in size called metaphloem. The protophloem is short lived. It is crushed by the developing metaphloem. Phloem fibres are absent in the primary phloem of most of the angiosperms. But they are usually present in the secondary phloem.

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Reference: Tamil Nadu Biology, Class: XII, Unit 2: Plant anatomy, Page no: 87.

Explanation:

Viroids: In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight.

Viroids are the smallest known infectious agents. Shown to occur only in plants, they cause several important diseases of cultivated plants, including potato spindle tuber, citrus exocortis, chrysanthemum stunt, and cucumber pale fruit. Some viroids are made up of circular single stranded RNA; others are linear single stranded RNA molecules. Each contains between 270 and 380 nucleotides.

Reference: NCERT Biology Text, Class: XI, Unit I: Diversity in the Living World, Chapter 2: Biological Classification & Microbiology: Application Based Approach By Pelczar, Page no: 430.