但网络的最边缘——叶子，却具有不同的结构，与木质部和韧皮部不同，叶子的纹路交叉并成环。
But the very end of such a network, the leaf, has a different architecture. Unlike the xylem and phloem, the veins in a leaf cross-link and loop.
在老茎中，仅最新的木质部环参与物质的运输。这些木质部环组成边材。
In older stems, only the most recent rings of xylem participate in the transport of materials. these make up the sapwood.
这样的加厚允许木质部在环间沿展，当周围组织生长时，不会破裂。
Such thickening allows extension of the XYLEM between the rings so that it is not ruptured as the surrounding tissues grow.
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