22 2 Alexander W. Evans, 



tacle proper), which bears the sexual organs. In some cases the 

 disc shows clearly that it has but one plane of symmetry. In 

 other cases it presents the appearance of being radial ; but even 

 here, as recently emphasized by Goebel," there is actually but a 

 single plane of symmetry, a fact made clearly evident when the 

 structure and development of the receptacle are considered. The 

 stalk, likewise, looks superficially as if it were radial, but here 

 again a single plane of symmetry is present, and the stalk main- 

 tains its dorsiventrality (or zygomorphy) in spite of its erect 

 position. 



It has already been noted that the receptacles represent pro- 

 longations of prostrate branches. These branches may be more 

 or less elongated, but they are often very short, a receptacle 

 being developed almost immediately after a dichotomy has taken 

 place. A receptacle, as shown so clearly by Leitgeb,^^ is a branch- 

 system, the growing point of the original prostrate branch under- 

 going one or more divisions. A study of the stalk shows that the 

 first division usually takes place very early in the development 

 of the receptacle. If a cross-section is examined (Figs. 5, K; 8, 

 5; etc.) the dorsiventrality of the stalk becomes at once 

 apparent, and the side which represents the ventral por- 

 tion usually shows two deep longitudinal furrows, enclosed 

 by scales and containing tuberculate rhizoids, the dorsal side 

 being destitute of such furrows. In very rare cases a single 

 furrow is present near the base of the stalk (Fig. 20, I). 

 The presence of two furrows is evidence that the grow- 

 ing point has already divided once, even if the stalk itself 

 remains undivided. Usually no further divisions take place until 

 the disc begins to develop, but in some cases the stalk shows three 

 or four rhizoid furrows, indicating that one or two secondary divi- 

 sions have occurred. This is seen clearly in M. breviloba and M. 

 domingensis (Figs. 9, I-K; 12, A, D). In the first the stalks 

 of both male and female receptacles show four furrows apiece; 

 in the second the stalk of the male receptacle which is figured 

 shows three furrows, the stalk of the female receptacle show- 

 ing four. The occurrence of more than two furrows has 

 apparently been rarely observed in Marchantia and allied 



^ Organographie der Pflanzen, 2d ed. 686. 1915. 

 '° See Unters. iiber Leberm. 6: 20-37. 1881. 



