2 24 Alexander W . Evans, 



present and in M. cJienopoda, four, although deviations from 

 these numbers are of frequeiit occurrence. Goebel*'' considers 

 that the number of rays developed is dependent on nutritive con- 

 ditions. In an unnamed species from the Fiji Islands, related 

 to M. geminata, he notes a reduction in the number of rays to 

 two, showing' that only one dichotomy has taken place, and he 

 compares this extreme condition with the two-rayed female 

 receptacles found in Exormotheca and Aitchisoniella. 



The rays are in one plane and vary greatly in length. They 

 are sometimes much shorter than the undivided portion of the 

 receptacle, appearing in the form of rounded marginal scallops 

 separated by shallow but acute sinuses. This condition is seen 

 clearly in M. polymorpha and its allies. It is much more usual, 

 how^ever, for the rays to be longer than the undivided portion, the 

 whole receptacle thereby acquiring a palmate appearance. This 

 type of receptacle is found in such species as M. chenopoda and 

 M. domingensis and is commonly associated with a smaller number 

 of rays than the first type. In some cases at the tip of a ray a 

 slight depression marking the position of a growing point can 

 be discerned, even in an old receptacle, but often all traces of the 

 growing points disappear. The stalk is not attached to the disc 

 marginally but peltately, although often excentrically. The 

 peltate attachment is due to intercalary growth taking place in 

 the region where the dorsal surface of the stalk and the dorsal 

 surface of the disc would naturally be continuous. In this way 

 a thin plate of tissue is formed between the two external rays 

 of the disc, which would theoretically be distinct to their junction 

 with the stalk. The presence of this plate, similar in all essen- 

 tial respects to the tissue forming the sinuses, intensifies the radial 

 appearance which the receptacles of certain species show. 



In its structure the disc shows many of the features which are 

 found in the vegetative thallus. It is distinctly dorsiventral and 

 the differentiation into epidermis, photosynthetic tissue and com- 

 pact ventral tissue is clearly marked. On the ventral surface 

 of the rays scales with appendages and scales without appendages 

 can be distinguished in two or more series, and the appen- 

 dages are much like those of the ordinary scales except that they 

 are smaller and sometimes less constricted at the base. Rhizoid 



" Organographie der Pflanzen, 2d ed. 699. /. 669 II. I9i5- 



