622 Howe: Structural dimorphism in Galaxaura 



cortex in the "Cameratae" consists chiefly of large obviously 

 confluent intercellular chambers filled with lime, while in the 

 "Spissae" this middle layer is made up chiefly of rounded calcified 

 cells. Expressed in another way, it may be said that the two 

 outer layers of the cortex in the "Cameratae" are filamentous, 

 if two cells in a row (or, often, two or three cells supported 

 capitately by a single stalk-cell) may be said to constitute a 

 filament, while in the "Spissae," the cortex is parenchymatous or 

 subparenchymatous throughout. In the process of decay or on 

 teasing to pieces with needles after decalcification, it often happens 

 that the outer cortex peels off, leaving the inner cortex still sur- 

 rounding the central strand of filaments. In the "Cameratae", 

 such a separation commonly takes place near the base of the 

 stalk-cells constituting the middle layer, so that the two outer 

 layers are exfoliated together, leaving the stumps of the stalk-cells 

 protruding from the large firmly coherent cells of the inmost layer. 

 In the "Spissae," the separation under such circumstances takes 

 place between the two outer layers, only the single superficial 

 layer being exfoliated, the middle layer of rounded subparenchym- 

 atous cells remaining firmly attached to the larger cells of the 

 inmost layer. 



In the course of a microscopic examination of certain specimens 

 of Galaxaura from Bermuda, Florida, and the West Indies, repre- 

 senting forms currently referred to G. obtusata, it was noted that 

 some of these specimens showed the cortex structure of the 

 "Cameratae" group while others showed that of the "Spissae" 

 group. It was at first supposed that two or more species were 

 represented in this material, but it was afterwards observed that 

 the two forms were often collected together almost throughout 

 their range, that they showed the same or parallel variations in 

 external characters, and that they could not be separated without 

 a microscopic examination. A little later it was noted that 

 whenever reproductive organs could be found (and by search 

 they could be found in most of the specimens), the plants of the 

 "Cameratae" structure were always tetrasporic, while those of 

 the "Spissae" structure were always antheridial or cystocarpic. 

 This discovery led to a careful reexamination of all the available 

 material, with results that were confirmatory of this correlation. 



