The vascular bundles and cellular tissue are sometimes developed in such a way as to form a circle, with a hollow in the centre, and thus give rise to what are called fistular or hollow leaves, as in the onion, and to ascidia or pitchers.
In many cases internal casts have been formed, some large cavity, such as a fistular pith, having become filled with mineral substance, which has taken the impress of the surrounding structures, such as the wood.
Fistular and partitioned by frequent diaphragms, as in some species of Pinus and other plants at the present day.
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