Why dicot leaves have recticulate venation?
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Answer:
Reticulate venation is not uncommon in monocots. Several members of Araceae, for example, Xanthosoma sagittifolium, Cyrtosperma johnstonii; Dioscoreales, eg. Chinese yam; Smilacaeae, eg. Smilax melastomifolia, etc. have reticulate venation. I haven't seen a cross section of the reticulate monocot leaves to know if they have distinct palisade and spongy mesophyll cells. But, the difference between palisade and spongy is less distinct in some dicots with the same number of stomata on both the adaxial and abaxial side. However, there are flowering plants that do not exactly fit into either monocots or eudicots. Amborella trichopoda and water lilies are now considered to be basal angiosperms and are classified separately at the base of the angiosperms although A. trichopda was included with dicots and water lilies with monocots in previous classification systems. A. trichopoda and water lilies have reticulate venation but their vascular system lacks vessels. It looks like to me that the dicot leaf is more ancestral than the monocot leaf, which suggests that reticulate venation in monocots may indicate a basal lineage in monocot evolution. I would be interested to hear if somebody has experience with any of the basal angiosperms.