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Title: Scaling of phloem hydraulic resistance in stems and leaves of the understory angiosperm shrub Illicium parviflorum
Premise of the Study

Recent studies in canopy‐dominant trees revealed axial scaling of phloem structure. However, whether this pattern is found in woody plants of the understory, the environment of most angiosperms from the ANA grade (Amborellales‐Nymphaeales‐Austrobaileyales), is unknown.


We used seedlings and adult plants of the understory tropical shrubIllicium parviflorum,a member of the lineage Austrobaileyales, to explore the anatomy and physiology of the phloem in their aerial parts, including changes through ontogeny.

Key Results

Adult plants maintain a similar proportion of phloem tissue across stem diameters, but larger conduit dimensions and number cause the hydraulic resistance of the phloem to decrease toward the base of the plant. Small sieve plate pores resulted in an overall higher sieve tube hydraulic resistance than has been reported in other woody angiosperms. Sieve elements increase in size from minor to major leaf veins, but were shorter and narrower in petioles. The low carbon assimilation rates of seedlings and mature plants contrasted with a 3‐fold higher phloem sap velocity in seedlings, suggesting that phloem transport velocity is modulated through ontogeny.


The overall architecture of the phloem tissue in this understory angiosperm shrub scales in a manner consistent with taller trees that make up the forest canopy. Thus, the evolution of larger sieve plate pores in canopy‐dominant trees may have played a key role in allowing woody angiosperms to extend beyond their understory origins.

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Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
American Journal of Botany
Page Range / eLocation ID:
p. 244-259
Medium: X
Sponsoring Org:
National Science Foundation
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