NUMBHR 40 • 17 



FIGURE 11. Horizontal fracture in a specimen of C. conipactiiDi collected in summer of 20 1 0 (Great Northern Peninsula of Newfound- 

 land). (A) The narrow bench to the left is the meristem fracture zone. (B) The main fracture bench (center in A) occurred on a plane with 

 a high level of cell fusion in the autumn of 2009 (when sporangial development initiated). Also visible is the transition from summer to 

 winter in perithallial tissues. 



Although degenerate (broken-out) or faint ghosts of growing-in 

 sporangial conceptacles were sometimes present in the remaining 

 22 specimens and buried sporangial conceptacles were present 

 in section, no mature, multipored sporangial conceptacles were 

 seen at their surfaces. In section, the most recent band of buried 

 conceptacles occurred about 100 pm below the meristem, indi- 

 cating maturity the previous winter. 



Both specimens collected in December presented premature 

 to mature sporangial conceptacles (Figure 13 A). It is not usually 

 possible in SEM images to distinguish between bisporangia and 

 tetrasporangia in buried conceptacles (the latter likely indicat- 

 ing meiosis and reduction to the haploid state in the spores). 

 However, both December specimens presented tetrasporangia, 

 and Lebednik (1973) found only tetrasporangial conceptacles in 

 this species. Although the data are minimal, this finding suggests 

 that C. nereostratiim generally has the triphasic and isomorphic 

 alternation of generations described earlier. In this case, sporan- 

 gial (2n) plants reproduce in autumn, maturing in winter (as in 

 C. compactiim in the Gulf of Maine and Nova Scotia) and gam- 

 etangial (In) plants reproduce in spring, maturing in summer. 



On the basis of available collections, gametangial plants are 

 probably infrequent in C. nereostratiim (very extensive collec- 

 tions demonstrate rarity in C. cunipactiim). Thus, especially in 

 C. nereostratiim, it is essential to identify the type of conceptacle 

 (sporangial or gametangial) in order to determine the develop- 

 mental timing of the conceptacle compared to the vegetative tis- 

 sue. Male and female conceptacles are quite small compared to 

 sporangial conceptacles; although both become buried, they are 

 not likely to be confused for sporangial conceptacles (Figures 7, 

 13). Fertilized female conceptacles (producing carposporangia) 



are somewhat larger in diameter than sporangial conceptacles 

 (Figures 7, 13). However, they are not as tall, suggesting that 

 carposporangial tissue dissolution is not as extensive beneath the 

 conceptacle. Since carposporangia are produced only laterally 

 from fertilized carpogonia (egg cells), centrally there is no disso- 

 lution, resulting in the formation of a central columnella, leading 

 to dumbbell-shaped conceptacles. For a detailed description of 

 the development of sexual conceptacles in C. nereostratiim, see 

 Lebednik ( 1976). Carposporangial conceptacle development has 

 not been described in C. compactiim, but it is likely quite simi- 

 lar. The presence of a central columnella and dumbbell-shaped 

 conceptacles shows that it is the developing sporangia (bisporic, 

 tetrasporic, or carposporic), not the vegetative cells, that provide 

 the acid for surrounding carbonate dissolution. 



Female conceptacles are located midway through the yearly 

 cycle (Figure 13B,C), with a sharp perithallial boundary in late 

 autumn (see below). On the other hand, sporangial conceptacles 

 are located near the upper (later) boundary of the cycle (Fig- 

 ure 13A). In medial sections, sporangial conceptacles are oval in 

 shape, whereas female conceptacles are dumbbell shaped, with 

 a central columnella. In sporangial conceptacles, the recalcified 

 sporangial walls are vertical, whereas the female conceptacle car- 

 posporangia and supporting structures appear as packed spheres 

 of many sizes (Figure I3B). 



Cellular and Anatomical Structure 



Aside from a basic pattern of upright filaments of cells 

 ct)mbining to form a calcified mat showing vague "rings," the 

 perithallial vegetative anatomy of Clathromorphum, as seen in 



