HYMENOPHYLLACEAE 581 
than in Hymenophyllum, while on the other hand the receptacle is usually 
longer, and has more continued intercalary growth, the question arises whether 
the larger number of sporangia will approximately compensate for their lower 
individual output. A computation was made of the output per sorus in 
Hi. Tunbridgense and dilatatum, and compared with a similar computation 
in Z. rentforme and radicans, with the result that, notwithstanding the great 
variations in spores per sporangium, the output per sorus appears approxi- 
mately uniform for the cases quoted. Thus the increased length of the 
receptacle and higher number of the sporangia tends to compensate the 
smaller output per sporangium which is seen 
in its extreme form in some species of 
Trichomanes. 
It has been seen that in Gleichenia, where 
there is a median dehiscence of the sporangium, 
elbow room is required for the process of 
mechanical ejection of the spores, and that this 
is only possible where the sporangia are loosely 
arranged. In a crowded sorus such as that of 
the Hymenophyllaceae the necessary space is 
not available. In Zoxsoma this difficulty is met 
by the incomplete development of the annulus, 
which then merely opens the distal end of the 
sporangium and allows the spores to be shaken 
out. But in the Hymenophyllaceae the whole 
mechanism is altered by the adoption of the 
oblique lateral dehiscence: as this is usual in 
the basipetal sori of the Gradatae it demands — , Uigeren, lipstrating the relative 
. < receptacle in the Hymenophyl- 
the greater attention. The sporangia are placed = [RCePrAaCtg ene nstated tom 
relatively to one another as seen in the diagram Se 
(Fig. 327), in which the cells of the annulus 
traversed in’ the median section of the sporangium are indicated by 
heavier lines. It is plain that on dehiscence taking place laterally, the 
distal side of each annulus has freedom to alter its form independently 
of the adjoining sporangia:! the oblique position of the annulus thus 
finds a practical explanation, and is even a necessity where the sporangia 
are short-stalked and imbricate. The dehiscence is, however, aided by its 
occurring usually in strict basipetal sequence in the Hymenophyllaceae : 
after the lateral slit is formed, the annulus is first of all reflexed, and then 
recovers with a sudden jerk, which often dislodges the whole sporangium, 
and scatters the spores. The result in Z+richomanes is that the distal 
end of the receptacle may remain bare of sporangia, an indication of the 
extent of the intercalary growth, while young sporangia may still be found 
around its base. 
Fic. 327. 
+As a matter of fact the freedom is greater than is shown in the diagram, for the sporangia 
alternate, and are not disposed in closely consecutive orthostichies. 
