## 2 Phylogenetic Variations in Cd Accumulation in Plants
They have played important roles in early land plant evolution and been remarkably adapted to a wide range of environments, including both tropical and cold temperate climates, alpine and lowland regions, and aquatic and xeric conditions [28]. In general, although both ferns and lycophytes can accumulate large amounts of aluminum (Al) and arsenic (As), they accumulate much less Cd [29; 30]. However, [PERSON] et al. [31] measured the Cd concentrations in two _Azolla_ species (small aquatic ferns) and found that _Azolla pinmata_ would accumulate 4673.8 mg/kg Cd and _Azolla filiculoides_ would accumulate 2747 mg/kg Cd when exposed to 500 uM of CdCl\({}_{2}\) for 3 days, thereby suggesting the potential of _Azolla_ species in Cd hyperaccumulation. The Gymnosperm clade is ancient and widespread, and it includes four of the five main seed plant lineages, including cycads, ginkgos, genophytes, and conifers [32]. Gymnosperm lineages diverged from each other during the period from the Late Carboniferous to the Late Triassic (311-212 Mya), earlier than the appearance of the earliest extant angiosperms (around 300 Mya) [33]. Living gymnosperms consist only of a little more than 1000 species, with conifers (pines, cypresses, and relatives) being the largest group [34]. The available researches on Cd accumulation in gymnosperms are mostly focused on conifers [35; 36; 37; 38; 39; 40; 41; 42]. [PERSON] et al. [35] reported that the Cd accumulation in the shoots of _Pinus sylvestris_ could amount to about 33.2 mg/kg when plants are treated with 10 mg/kg Cd for 11 weeks. In a subsequent study, however, another Pinus species, _Pinus pinaster_, was found to accumulate only 11.9 mg/kg Cd in its shoots when exposed to 15 mg/kg Cd for six months [36]. [PERSON] and [PERSON] [37] found that _Picea abies_ grown near the metal production factory (Cd at the depth of 0-10 cm beneath the level of the soil: 6.8 mg/kg) accumulates 1.1 and 1.5 mg/kg Cd in its leaves and stems. On the other hand, [PERSON] and [PERSON] [38] found that _Picea abies_ could accumulate up to 72.4 mg/kg and 26.1 mg/kg Cd in its barks and wood, respectively, after being exposed to 0.5 uM Cd for three months. [PERSON] et al. [39] showed that _Platycladus orientalis_ (containing 28.0, 69.3, and 406 mg/kg Cd in leaf, stem, and root, respectively) accumulated a 2.30-fold higher amount of Cd in leaves compared with _Juniperus chinensis_ (containing 12.2, 49.4, and 455 mg/kg Cd in leaf, stem, and root respectively), after being exposed to 100 mg/kg of Cd for 220 days. Recently, [PERSON] et al. [PERSON] et al. (2018) reported that the growth of _Platycladus orientalis_ was significantly inhibited in soil containing 9.6 mg/kg Cd, while it accumulated more than 2 mg/kg and approximately 41.5 mg/kg Cd in its stems and roots, respectively. Although the Cd accumulation results were not consistent between the above two studies ([PERSON] et al., 2018; [PERSON] et al., 2018), the authors of both studies suggest that _Platycladus orientalis_ is a promising plant for the phytostabilization of Cd-contaminated soil. In addition, the hybrid larch was found to show an extraordinary potential for the phytoextraction of Cd from the contaminated soils to the above-ground tissues, as it could tolerate a 4-week exposure to 0.25 mM Cd (or a 1-week exposure to 1.5 mM Ca) and accumulate as much as 208-220 mg/kg Cd in its shoots ([PERSON] et al., 2018; [PERSON] et al., 2018). Angiosperms first appeared in northern Gondwana during the Early Cretaceous (approximately 135 Mya) and dominated the species composition worldwide within 10-30 Mya ([PERSON] et al., 2018). They are the largest and most diverse group within the plant kingdom, representing approximately 80% of all known living plants and including five major groups: eudicots, monocots, magnoliids, chloranthales, and ceratophyllales ([PERSON] et al., 2018). Angiosperms are able to occupy any environment on earth, such as high mountaintops, deep oceans, freezing tundras, and warm, wet rainforests.