/ 



42 L. HILLIS- COLIN VAUX 



In several species the basal point of attachment is poorly developed 

 and may be difficult to locate, particularly in dried material. Instead, 

 where some of the branches touch the substrate fine rhizoidal filaments 

 may develop, giving attachment. This pattern may occur together with a 

 spreading type of growth where thallus branches along crevices, or over 

 and around coral heads. Adventitious holdfasts occur in some plants 

 oi opuntia (Fig. 19), distorta, gracilis and copiosa. 



(c) Habit and growth form. The habit of the various species may be 

 erect, pendant, prostrate or spreading, and may be distinctive for 

 certain species such as the flaccid straggling thalli of the typical 

 gracilis, the pendant form of a number of species of the Halimeda 

 section (Section IV), or the generally erect pattern of members of the 

 Rhipsalis. The axis of new growth is predominantly vertical for thalli 

 with erect or pendant habits, horizontal for those that are prostrate or 

 spreading. Some members of the section Opuntia, with their sprawling 

 habit, may exhibit both horizontal and vertical axes of strong growth. 



2. Microscopic characters 



Three main sets of microscopic characters are useful in the 

 taxonomy of Halimeda. They are : 



pattern of medullary filaments at the node (Table III ; Figs 3, 15) ; 

 size and appearance of primary, secondary and tertiary utricles 

 (Figs 17, 20); 



pattern of cortex, and extent of its development (Fig. 20). 



The use of these characters almost always requires the magnification 

 of low and high powers ( x 10 and x 40) of a compound microscope, 

 preceded by selection and preparation of the material. 



(a) Choice of segments for examination. In using these characters to 

 identify Halimedae, it is important to select only mature segments 

 from the plant. In so doing much of the variation which is the product 

 of growth rather than of the individual species is avoided. The ideal 

 segment for examination is free of epiphytes and usually is about the 

 middle of the thallus. Material from such a location is nearly always old 

 enough to be reasonably calcified, but has not developed various 

 features of aged or senescent segments which could mislead. The 

 variations I describe herein and the range of measurements given in 

 this section and in Section IV are from such "mature" segments. Some 



