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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.