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Fig. 7.5-4a. Transverse section of developing vessels in roots of oak (Quercus). This bundle contains many mature vessel elements, readily identifiable by their large diameter and red-stained, lignified secondary walls. But the cells marked with arrows have the proper size to be vessel elements but they do not have secondary walls. Also, there is a bit of plasmolyzed protoplasm in them. These are vessel elements that are still differentiating – they appear to have finished their enlargement and had not even begun to deposit the S₁ layer of the secondary wall. It is not too common to see vessel elements in this immature stage of development, they seem to pass through this stage quickly.