Fig. 1.

(A) The relationship between shoot Mg concentration ([Mg]_shoot) and shoot Ca concentration ([Ca]_shoot) among 212 eudicot species (black and orange circles), of which 61 were members of the Caryophyllales order (orange circles). The lines are predictions for the relationships between [Mg]_shoot and [Ca]_shoot for non-Caryophyllales eudicots (solid line) and Caryophyllales species (broken line) from the model based on leaf anatomy described in the text and the data presented in Table 1. (B) The relationship between [Mg]_shoot and [Ca]_shoot among 76 commelinid (green circles) and 35 non-commelinid monocot species (blue circles). The original data set contained three non-commelinid species with [Ca]_shoot >50 mg g^–1 DM that are not plotted. The line is a prediction for the relationship between [Mg]_shoot and [Ca]_shoot for commelinid monocots from the model based on leaf anatomy described in the text and the data presented in Table 1. (C) The relationship between [Ca]_shoot and the cation exchange capacity (CEC) of root cell walls of 44 angiosperm species, comprising 16 commelinid monocots (green circles), one non-commelinid monocot (blue circle), five Caryophyllales (orange circles) and 22 other eudicots (black circles). (D) The relationship between [Mg]_shoot and the CEC of root cell walls of the same 44 angiosperm species. The [Ca]_shoot and [Mg]_shoot for angiosperm species are mean values obtained in the six hydroponic experiments described by White et al. (2017) and collated by Neugebauer et al. (2018). Values for the CEC of root cell walls were obtained from the literature survey of White and Broadley (2003), and are estimated means from a REML analysis of the data.