THE ADULT SPOROPHYTE 



115 



size of the sporangium and the fact that it has a well-marked though short pedicel. 

 In the simplest forms of Botrychium, such as B. simplex, the sporangia are much 

 larger and are sessile. 



In B. virginianum (fig. 85) the development of the individual sporangia begins 

 just about a year previous to their ripening, and if the plants are taken up about the 

 time the spores are shed the earliest stages of the sporangia will be found in the leaf 

 which IS to expand the following season. At this time the sporangiophores in the 

 larger specimens are thrice pinnate, and the youngest recognizable sporangia are 

 borne at the tips of the branches. These young sporangia form slight elevations 

 which become smaller as they approach the tip of the segments, and if an exact 

 median section is made of one of these young sporangia it will be seen to have at its 

 apex a large pyramidal cell with a truncate base. Holtzman (Holtzman 1), states 

 that the whole sporangium may be traced back to a single cell and that the divisions 

 at first are like those of a three-sided apical cell. I have not been able to satisfy 

 myself as to the accuracy of this statement, but the youngest stages which I have 

 been able to find would not forbid such an interpretation, although there seems no 

 question that the basal part of the sporangium is derived, in part at least, from the 

 surrounding tissue. 



Fig. 86. 



A. Sporangiophores of Helmittlhoslachys. The two lower figures seen from above. /, sterile appendages. X3. 



B. Section of a young sporangium, showing sporogenous tissue, 3p; tapetum, i; and wall, ti-. (From preparation 



made by Prof. L. L. Burlingame.) 



The terminal cell of the sporangium is divided by a periclinal wall into a single 

 inner cell which forms the archesporium, and an outer one which contributes to the 

 wall of the sporangium. The outer cell rapidly divides and similar divisions occur 

 in the archesporium. By active growth in the basal part of the sporangium it pro- 

 jects more and more until it assumes the form of a projecting globular body with a 

 short stalk, thus differing strikingly from the deeply sunken sporangia of Ophio- 

 glossum. It is impossible to detect any definite succession of divisions in the arche- 

 sporium, which ultimately becomes changed into a large globular mass of cells with 

 the usual dense contents, separated from the outside of the sporangium by half a 

 dozen or more layers of sterile cells. The ceils immediately adjoining the sporo- 

 genous tissue constitute the tapetum. Bower states that the tapetum is exclusively 

 derived from this outer sterile tissue, but it must be said that, in Botrychium vir- 

 ginianum at least, the limits between the sporogenous tissue and the tapetum are by 

 no means easy to detect. The wall of the ripe sporangium has from four to six 

 layers of cells and the dehiscence is by a vertical cleft, whose position may sometimes 



