XXVII] 



SPORANGIA 



243 



Though apparently a minor detail this segmentation is actually an important 

 point for comparison. 



The sporangium projects horizontally from the surface of the receptacle, 

 with the annulus directed obliquely towards the apex of it. The effect of 

 this on the process of dehiscence is suggested by a dia- %^ 

 gram constructed from a section of a single sporangium 

 of Tricho7nanes speciosuni taken by Prantl (Fig. 515). 

 Three such sporangia are represented one above another : 

 the cells of the annulus traversed in the section are 

 indicated by heavier lines. It is plain that on dehiscence 

 taking place laterally, the distal side of the annulus has 

 freedom to alter its form independently of the others, 

 and it straightens itself out after the rupture. The 

 oblique position of the annulus and its lateral dehiscence 

 thus find their practical explanation, and this arrange- 

 ment is even a necessity w^iere all the stalks are short, 

 and the sporangia overlap. Further, the dehiscence is 

 aided by its occurring in the sporangia in basipetal 

 sequence: for after the lateral slit is formed, the annulus 

 is first reflexed, and then it recovers with a sudden 

 jerk which often dislodges the whole sporangium. In 

 Trichomanes the distal end of the receptacle may be 

 thus stripped of its sporangia, while a succession of 

 young sporangia may still be found around its base. 



The spore-output per sporangium is extremely variable in the family, as 

 the following table shows: 



Fig. 515. Diagram illus- 

 trating the relative posi- 

 tion of the sporangia on 

 the receptacle in the 

 Hymenophyllaceae. It 

 was constructed from 

 Prantl's section of a 

 mature sporangium of 

 Trichomanes speciosum . 



The limits of variation in number are here wider than it is found to be in 

 an}- other family: ranging between such numbers as 421 and 32. The 

 number of spores runs fairly parallel with the size of the sporangia. But on 

 the other hand where the numbers are small there ma)- be a correlati\-e 



16—7 



