In both isobilateral and dorsiventral types of leaf, xylem is present near the upper (adaxial) epidermis and phloem towards the lower (abaxial) epidermis. Why is this so?
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Establishment of the dorsoventral polarity of the leaf is under the control of a large array of regulators, including, but not limited to proteins pertaining to various transcription factor classes.
To summarize (read more here):
One aspect of leaf development that is of particular interest is the establishment of dorsoventral polarity: the distinct adaxial (upper) and abaxial (lower) sides of the leaf. Studies over the past decade have defined genetic details of this model, and uncovered diverse mechanisms of gene regulation that facilitate development of leaf dorsoventral polarity, including transcriptional regulation, chromatin modification, DNA modification, regulation by short RNAs and translational and post-translational regulation.
REV/PHB/PHV transcriptional factors promote adaxial fate, while the identity of cells in the abaxial domain depends on the expression of KANADI transcription factor (source).
Vascular development in turn, regulated:
Central regulators for vascular development involves the REV/PHB/PHV/CAN/ATHB8 genes which are members of HD-ZIP III family and KAN (KANADI). These regulators act antagonistically to maintain xylem and phloem, respectively.
Also, notably:
Auxin plays an essential role in regulating vascular formation through PIN1 transporter by early markers like ATHB8.
PIN1 mediates auxin transport and manifests polarized localization on cellular membranes, while:
Phloem differentiation is induced by low-grade auxin stimulation, whereas xylem differentiation requires higher-auxin levels.
Thus, the mechanisms of leaf vascularization are highly complex (I merely touched them) and require a high understanding of Biology (as do the provided sources).