. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. HYMENOPHYLLACEAE 58i 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 H. Tunbridgense and dtlatatum, and compared with a similar computation in T. reniforme and radicans, with the result that, notwithstanding the great variations in spores per sporangium, t
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. HYMENOPHYLLACEAE 58i 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 H. Tunbridgense and dtlatatum, and compared with a similar computation in T. reniforme 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 Loxsoma 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 the greater attention. The sporangia are placed relatively to one another as seen in the diagram (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:1 the oblique position of the annulus thus finds a prac
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