Compared with traditional palaeoecological indicators such as pollen and spores, the herbaceous diagnostic phytolith morphotypes have more explicit representations for many plant groups, particularly for those in Poaceae including Bambusoideae, Oryzoideae, Arundiaceae, Chloridoideae and Panicoideae, and Cyperaceae [48] , etc. This advantage of herbaceous phytoliths has been widely applied to the reconstruction of grassland vegetation, and recovered details about the changes in vegetation composition and plant diversity of grassland at different geological time scales [6] [8] .

Phytolith types from Bambusoideae included Saddle, Bulliform flabellate, Blocky, Rondel, Stomata, Acute bulbosus, Tracheary, etc., among which Saddle (length/width ratio > 1) ( Figure 1(a) ) and Bulliform flabellate (clavate margin) ( Figure 1(b) ) were diagnostic morphotypes of Bambusoideae [2] [25] . Phytoliths from Oryzoideae included Bilobate, Bulliform flabellate, Rodel, Elongate sinuate, Acute bulbosus, Double-peaked glume, etc., among which Bulliform flabellate with scale-decoration ( Figure 1(c) ) and Double-peaked glume ( Figure 1(d) ) were diagnostic morphotypes [31] [49] - [53] . Phytolith from Arundiaceae included Saddle, Bulliform flabellate, Elongate sinuate, Trapzoid, Acute bulbosus, etc., among which Saddle (length/width ratio ≈ 1) ( Figure 1(e) ) and Bulliform flabellate (shield margin) ( Figure 1(f) ) were diagnostic [2] [27] [41] . Phytoliths from plants of Chloridoideae included Saddle, Bilobate, Elongate sinuate, Acute bulbosus, Bulliform flabellate, Tracheary, etc., among which Saddle with a length/width ratio < 1 ( Figure 1(g) ) was the diagnostic morphotype [2] [21] . Phytolith from Pooideae included Bilobate, Elongate dentate, Rodel, Acute bulbosus, blocky, Tracheary, etc., among which Bilobate trapeziform ( Figure 1(h) ) and Elongate dentate ( Figure 1(i) ) were diagnostic morphotypes [2] [26] . Phytolith from Panicoideae included Bilobate (including Cross), Polylobate, Blocky, Elongate sinuate/Elongate dendritic, Bulliform flabellate, Rodel, Acute bulbosus, Tracheary, etc., among which Bilobate ( Figure 1(j) ), Polylobate ( Figure 1(k )) [2] [54] and Elongate sinuate of lemma ( Figure 1(l) ) [3] [4] [5] were diagnostic morphotypes. Cyperaceae phytoliths included Polygonal granulate ( Figure 1(m) ), Papillate ( Figure 1(n) ), Acute bulbosus, Elongate, Stomata, Tracheary, etc., among which the first two were the diagnostic morphotypes [2] [42] [55] . In addition, there were some other phytolith producers in herbaceous plants (e.g., Zingiberaceae [39] , Commelinaceae [56] ).

In recent years, phytolith morphological studies of woody plants have gradually established diagnostic phytolith morphotypes for the groups of broadleaved trees and conifer trees, which exhibit significant potential in the reconstruction of forest and woodland environment [28] [40] [44] [46] [47] . It was found that phytolith types in broadleaved trees included Stomata stellate, Elongate brachiate geniculate, Irregular sinuate, Polygonal tabular, Trichome irregular tubercule, Trichome bulbous irregular, Elongate facetate, Tracheary annulate/facetate geniculate, Tracheary annulate/facetate claviform, Tracheary helical, Spheroid favose, Elongate entire, Spheroid hollow, Irregular articulated granulate and so on [44] . Among these types, Elongate brachiate geniculate ( Figure 2(a) ), Polygonal tabular ( Figure 2(b) ), Elongate facetate ( Figure 2(c) ), Tracheary annulate/facetate claviform ( Figure 2(d) ) and Tracheary annulate/facetate geniculate ( Figure 2(e) ) could be used as diagnostic phytolith morphotypes for broadleaved trees [44] . Besides, Spheroid echinate ( Figure 2(f) ) was a diagnostic phytolith morphotype for Arecaceae [28] [39] [46] , whose diameter (6 - 25 µm) could be used

to distinguish it from Spheroid echinate of Bromeliaceae (<2 to 10 µmm) [39] [46] . The morphotypes of conifer phytoliths included Blocky polyhedral, Elongate tabular cavate, Elongate echinate, Elongate entire, Stomata, etc., among which Blocky polyhedral ( Figure 2(g) ) and Elongate tabular cavate ( Figure 2(h) ) were diagnostic morphotypes [40] [47] .

In addition, studies on modern fern phytoliths have shown that there are mainly 12 types of fern phytoliths, including Acicular, Amoeboid branchiate, Elongate echinate, Elongate sinuate, Irregular anticlinal, Prismatic elongate, Prismatic cubic, Stomata, Spheroidal cavate granulate, Silicified epidermis granulate, Tracheary elongate, and Silicified epidermis/hypodermis [45] [57] . Among them, Irregular anticlinal ( Figure 2(i) ), Prismatic elongate ( Figure 2(j) ), Elongate sinuate ( Figure 2(k) ), Spheroidal cavate granulate ( Figure 2(l) ), Silicified epidermis granulate ( Figure 2(m) ), Amoeboid branchiate ( Figure 2(n) ) and Prismatic cubic ( Figure 2(o) ) were diagnostic morphotypes for ferns [45] .

Researches in environmental archaeology have promoted investigation and application of diagnostic phytoliths of crops and their relatives [2] [3] [4] [5] [10] - [15] [53] . Some phytolith morphotypes have the potential of identification precision and accuracy to genus or species [3] [4] . Therefore, they have been widely utilized in the study of domesticated and cultivated processes of key crops in dryland and rice agriculture in East Asia, such as rice, various types of millets and their relative plants [4] [12] [14] [15] [16] .

Morphological studies of maize (Zea mays L.) phytolith confirmed that Cross ( Figure 3(a) ) was the diagnostic morphotype, whose size and basal features could be used to distinguish it from other grasses [1] [13] [58] [59] . The diagnostic phytolith morphotype of cereals were Epidermis elongate dendritic and Papillate ( Figure 3(b) ), whose size and margin shape could be used for identification of wheat and barley remains [17] [60] .

Rice (Oryza sativa) is one of the most important crops in East Asia [49] . Rice phytolith of Double-peaked glume ( Figure 1(d) ) and its morphological parameters could be used to distinguish between wild and domesticated rice [31] . The number of fish-scaled decoration on Bulliform flabellate ( Figure 1(c) ) could also effectively distinguish between wild and domesticated rice [50] . The percentage of Bulliform flabellate with ≥ 9 scale decorations among all Bulliform flabellate phytoliths could be used as an indicator to evaluate whether ancient rice had developed domestication properties [49] [50] [51] [52] [53] , based on the finding that the lowest value of Bulliform flabellate with ≥9 scale decorations was ~40% in domesticated rice [50] .

The glumes phytoliths of millets, typical crops of dryland agriculture in China, had important significance in identification of ancient millet plants [12] . The Ω-type ( Figure 3(c) ) and η-type ( Figure 3(d) ) margin shapes of Epidermis elongate dendritic on the outer and inner glumes were the most important

features for identifying common millet and foxtail millet, respectively [12] . Furthermore, Lu et al. [3] demonstrated five identification criteria of diagnostic morphological features for these two groups: 1) foxtail millet had Cross phytolith ( Figure 3(e) ) in glumes and lower lemma, while common millet had Bilobate ( Figure 3(f) ); 2) only foxtail millet had Papillae in upper lemma and palea ( Figure 3(g) ); 3) Elongate dendritic in upper lemma and palea of foxtail millet had Ω-type margins, while that of common millet had η-type margins, and these two margin types could be subdivided into three subtypes; 4) the ends of epidermal long cells of foxtail millet and common millet were cross-wavy type ( Figure 3(h) ) and cross-finger type ( Figure 3(i) ), respectively; 5) rippling feature developed on the surface keratose layer and long cells of the upper lemma in foxtail millet ( Figure 3(j) ). Moreover, Zhang et al. [30] [34] found that morphological parameters of Elongate dendritic of ΩIII margin on inner and outer glume could discriminate between foxtail millet and green foxtail. Ge et al. [33] found that the diagnostic phytolith morphotype of barnyard grass was Elongate dendritic with β-type margins ( Figure 3(k) ).

In addition, Wang et al. [35] found that Tabular sinuate verrucate ( Figure 3(l) ) and Tabular sinuate psilate ( Figure 3(m) ) were the diagnostic phytoliths in bastfiber crops. Recently, a new kind of calcium phytoliths, with similar anatomical origin to silicified phytoliths, were successfully applied in the identification of ancient tea residues [61] - [63] . The diagnostic morphotypes of tea calcium phytoliths in Camellia sinensis L. were Trichome base calcium phytolith with regular cracks ( Figure 3(n) ) and Druse calcium phytolith ( Figure 3(o) ) [61] [62] .