1960J 



ANATOMY OF GUAYANA XYRIDACEAE 



115 



central core of lignified parenchyma in upper portions is peculiar. The flattened 

 inflorescence axis, lacking veins in the "pith" region, is distinctive. Sepal 

 anatomy could be used to separate Achhjphila from the other genera, and floral 

 venation reveals both a lack of staminodia and a choripetalous condition. Pollen 

 is peculiar because it combines a peculiar patch-like presence of long pila with a 

 lack of major ornaments and a nonaperturate condition. 



Detailed comments cannot be made on Xyris, because it has been excluded 

 from this study. Such characteristics as lack of "pith'' bundles in the inflores- 

 cence axis, distinctive seed anatomy, and the peculiar nature of the monosulcate 

 pollen provide features which, as extended descriptions would show, amply 

 enforce the generic distinction of Xyris within the family. 



Composition of Xyridaceae : Relationships of the Four Genera. 



In respect to either gross morphology or anatomy, three relatively isolated 

 groups are apparent: (1) Abolboda and Orectanthe; (2) Achhjphila ; and (3) 

 Xyris. Before showing the anatomical common denominators among these 

 groups, similarities among pairs of genera may be mentioned. 



Abolboda and Orectanthe, as mentioned above, are amply distinct, but the 

 resemblances are close. For example, the presence of stjde appendages and their 

 venation pattern is a striking similarity. Winged seeds occur both in Orectanthe 

 and, to some degree, in Abolboda macrostachya. Seed-coat anatomy of the two 

 genera reveals two variations on the same basic pattern, as do the embryos. 

 The inflorescence axis in both genera contains bundles in the "pith" region. 

 Most significantly, perhaps, both genera possess pollen grains in which an outer 

 layer contains both minute pila and large spine- or knob-like ornaments. 



Abolboda. resembles Xyris in exhibiting a similar range of anatomical char- 

 acteristics in roots. Equitant leaves and compound bundles in leaves occur in 

 both genera. The embryo of Abolboda, although larger than that of Xyris, is 

 very similar. Better knowledge of seed anatomy in Xyris might reveal similar- 

 ities in this respect. 



Abolboda is like Achlyphila in relatively few aspects, because the relation- 

 ship of Achlyphila to Xyris appears closer. In addition to features common to 

 all genera mentioned below, one may mention resemblances in epidermal relief 

 in flowers, identical ovules, similarities in floral venation, and certain features 

 of root anatomy and inflorescence-axis structure. 



Relationships between Achlyphila and Xyris include those of equitant 

 leaves and their anatomical consequences, the presence of sclerified epidermis 

 and undulate epidermal contours on leaves, the lack of bundles in the "pith" 

 region of the inflorescence axis, and resemblances in pollen. The pollen of 

 both genera lacks the differentiation of sexine into major and minor ornaments, 

 and is relatively small in diameter. 



Review of anatomical characteristics reveals a number in which the genera 

 are alike, or in which the genera appear as variations on a common basis. One 

 significant feature is the presence of vessel elements with simple perforation 

 plates in all parts of the plant. As the work of Cheadle (1942) shows, this 

 high degree of specialization occurs in only a few families of monoeots. The 

 amount of similarity among the three genera in this study in respect to floral 

 venation is remarkable, and study of Xyris might enable inclusion of that 

 genus in this respect. The choripetalous condition in Achlyphila is really not 



