71 



as follows: doubling may arise either from the fertilization of 

 two eggs contained within one archegonium: or two archegonia 

 (or young sporophytes) may partially fuse during their early 

 development. The second explanation is, as Schimper has 

 pointed out, more likely and such is readily proved to be the 

 case in the specimen described below. 



Polytrichum commune L. 



The specimen was collected by the author on the high open 

 moorland in eastern Belgium about 10 kilometers north of 

 Malmedy (in the province of Malmedy), Belgium, in July, 

 1938. It is now in the Herbarium of the Missouri Botanical 

 Garden, St. Louis. (#1140623) 



The two thecae were enclosed in what appeared to be a 

 single calyptra and are shown in figure 1. One seta is somewhat 

 shorter than the other and its theca is correspondingly diminu- 

 tive (fig. 2). The sporophytes were not mature at the time of 

 collection which accounts for the rather shrunken and wrinkled 

 appearance of the young thecae after drying. Figure 4, a normal 

 mature theca, has been included for comparison. 



Examination of the setae has shown that the apparent fusion 

 is quite superficial and actually involves only the basal portions 

 of the old archegonia, the fringa at a (fig. 2) representing the 

 point at which the neck was torn from the remainder of one 

 archegonium. 



The most interesting feature of the specimen, however, is to 

 be found in the calyptra. Although slightly larger, it is externally 

 quite similar in size and form to those enclosing normal thecae. 

 Dissection of the calyptra, however, revealed the true nature of 

 its origin beyond a doubt, for instead of a single calyptra- 

 proper contained within the hairy covering, two are present 

 (fig. 3). In this figure the hairy cover has been split open to the 

 apex in order to show the distinctly unfused nature of the two 

 calyptra. 



The explanation of this phenomenon is then clear, and the 

 apparent doubling undoubtedly took place as follows: two ad- 

 jacent archegonia became superficially fused in the region of 

 their venters; as the setae elongated and began to carry aloft 

 their respective archegonial necks or calyptra the latter re- 

 mained sufficiently close together to be enclosed by a common 



