1960] 



ANATOMY OF GUAYANA XYRIDACEAE 



113 



present in these characters. In preparation of pollen grains in this species, the 

 knobs appeared to be sunken in slight depressions ; this may well be an artifact 

 of fixation or preparation. The knobs of Orectanthe sceptrum pollen grains are 

 different from those of 0. ptaritepuiana in that the apical portion is widened, 

 so that a capstan-like shape is achieved. The only variation in structure of these 

 knobs is found in the formation of additional small knobs on the base (fig. 127) 

 as compared to the normal condition (fig. 125), or the formation of a roughened 

 base (fig. 126). 



Discussion. 



In summary, the pollen grains of the three genera show excellent generic- 

 characters. The presence of large excrescences in Orectanthe relates this genus 

 closely to Abolboda, but the differing shape is a good generic character. Like- 

 wise, the larger pila in the surface between emergences differentiates Orectanthe 

 from Abolbocla. The absence of such large emergences in Achlyphila, and the 

 grouping of pila into flake-like aggregations on the exine surface, marks this 

 genus off from other genera of Xyridaccae. Within Abolbocla, differing exine 

 widths and distinctive spine shapes and sizes serve to differentiate the species, 

 so that of the species studied, most could be identified by means of the pollen 

 grains alone. Similar considerations apply to distinctions between the two 

 species of Orectanthe. 



Erdtman (1952) has emphasized the great distinction between Abolbocla and 

 Xyris in pollen-grain structure. Much of this distinction may be attributed 

 to the large spines on the surface of Abolbocla- pollen grains. The lack of such 

 larger ornaments on grains of Achlyphila, which are (like those of Abolbocla, 

 but unlike those of Xyris) nonaperturate, may vitiate this contrast, and pro- 

 vide an intermediate form. The pilate layer in pollen grains of Abolbocla, Orec- 

 tanthe, and Achlyphila probably has a counterpart in Xyris. The presence of 

 the pilate layer, although different in each genus, tends to provide a common 

 characteristic, as does the rather thick nexine, lacking in lipophilic compounds. 

 The writer suspects from Erdtman 's reports of both "OL" and "LO" sexine 

 patterns that a layer of united pila may compose the sexine in at least some 

 Xyris pollen grains. The exine of Achlyphila seems midway between that of 

 Abolbocla and Orectanthe on one hand, and Xyris on the other. The relatively 

 thick, baculate sexine and the thin nexine of Xyris pollen grains do not seem 

 at odds with the condition in Achlyphila. Probably the three major patterns 

 represent striking variations on the same basic pattern. Careful work is needed 

 both to provide additional data on Xyris and to demonstrate whether the affinity 

 Erdtman (1952) suggests between the grains of the Abolboda-tyipe and those of 

 Liliaceae and Zingiberaceae is close or not. 



SUMMARY 



The writer has found that an attempt to summarize anatomical character- 

 istics of genera of Xyridaceae in chart form is virtually impossible because (1) 



trum. Figs. 126-127. The same, variations in ornament shape. Fig. 128. Achlyphila disticha. 

 Fig. 129. The same, surface view of sexine ornamentation (pila). Scale = 5 



