ECOLOGY AND TAXONOMY OF Holimeda 45 



The most difficult categories are the second and third which involve 

 fusion of filaments in small groups, and they have sometimes been 

 considered subunits of but one category (Taylor, 1950; Hillis, 1959). 

 However, with the many additional species now placed in these 

 categories this treatment no longer seems appropriate. In spite of 

 apparent overlap, the two categories are distinct and should be 

 considered of equal importance to the others. It has been necessary, 

 however, to modify their definition. These new, or extended definitions 

 have been used in Table III which is an illustrated listing of the five 

 patterns, together with the species in which they are found. Modifica- 

 tions and variations are discussed below. 



(i) Further definition of second and third patterns of nodal filaments, 

 and their variations. In both categories fusion may be for a very short 

 distance (approximately 1-5 times the filament diameter) or may be 

 complete, with the participating filaments continuing as one filament 

 which subsequently branches. These sometimes are referred to as 

 "short" and "complete" fusion respectively. 



In both categories the fusion may involve units of up to six filaments, 

 and possibly more (Colinvaux, 1968a, and unpublished). 



In both categories the occasional filament may remain separate. 

 Such anomalous behaviour is perhaps most likely in filaments with a 

 peripheral position (Colinvaux, 1968a, and unpublished). 



Also, in certain species of both categories the fused units may adhere 

 laterally, giving the impression of fusion into large bundles. Such 

 adhesion may be demonstrated by teasing apart the units. This can be 

 done with care, and the filament walls remain intact (Hillis, 1959; 

 Colinvaux, 1968a). 



However, in species of the second category, which is delimited 

 (Table III) as "filaments mostly fuse in pairs for a short distance", 

 this is the predominant type of nodal filament pattern. In addition, 

 the participating filaments generally are not intricately intertwined 

 before fusion. Therefore, the pattern "definite short fusion in pairs by 

 most or all of the filaments, with participating filaments very little 

 entangled" is diagnostic, whenever observed, for the species listed, that 

 is, for the entire section Opuntia (Section IV). 



Category three now is delimited as "filaments fusing in small groups, 

 commonly twos or threes, for a short distance (approximately 1-5 times 

 the filament diameter) or completely; filaments are frequently mucn 

 intermeshed before fusing" (Table III). This category contains the 

 greatest number of species, is the most variable, and its species are 

 often the most tedious to dissect. The nodes may contain a more or less 

 equal mixture of "short" and "complete" fusion units, or completely 



