1891.] Morphology of Spore-producing Members. 



271 



horizontal rhizome), while the sporophylls (= " sterile fronds ") and 

 sporangia (= "fertile fronds") are largely extended, the elaboration 

 of the two showing a remarkable parallelism. In Ophioglossum vul- 

 gatum, where the sporophyll is simple though much larger than in 

 Lycopodium, the sporangium ("fertile frond") is elongated and 

 partitioned by the sterilisation of transverse bands of the potential 

 arehesporium. In Ophioderma (as also in some species of Ophioglos- 

 sum) the sporophyll is irregularly lobed, while the sporangium 

 ("fertile frond") is occasionally branched. In Helminthostachys the 

 sporophyll is branched, and the whole sporangium (" fertile frond ") 

 is also branched occasionally; but in addition it shows that elabora- 

 tion above described, and it may be looked upon as the result of 

 further partitioning of the arehesporium, and outgrowth of separate 

 parts of the superficial tissues as the " sporangiophores." Finally, in 

 some species of Botrychium the sporophyll and sporangium (" fertile 

 frond ") are both repeatedly branched, and show the furthest diver- 

 gence of the whole series from their simpler prototypes, which are the 

 sporophyll and sporangium of the Lycopods. 



In this view thus stated there is nothing incompatible with what 

 might be expected on a priori grounds ; on the contrary, there is 

 good reason to look upon such a progression as one of the natural 

 ways in which the number of spores produced might be increased, 

 such increase in number being obviously beneficial. Taking the 

 simple strobilus as the starting point, one method would be the 

 elongation of the strobilus, and increase in the number of sporangia 

 produced upon it, they remaining individually of relatively small 

 size ; this is the type seen in the Lycopods. A converse method 

 would be to increase the individual size of the sporangium, while the 

 number of sporangia might remain small and be matured at intervals ; 

 this is exemplified by the Ophioglossaceae. But if simple enlargement 

 of the sporangium took place, without subdivision of the arehesporium, 

 the rapid supply of nourishment to the enlarged mass of growing 

 spores would be difficult, while at the period when the sporogenous 

 mass is semi-fluid, owing to the cells separating from one another, and 

 floating freely in fluid, the sporangium would run great risk of 

 mechanical injury from without, and a single puncture of the wall of 

 the large sporangium would ruin the whole. These difficulties are 

 all avoided by partitioning of the sporangia ; the sterile tissue of the 

 partitions, while strengthening the whole, would, together with the 

 increased sub-archesporial mass, serve the more readily to bring 

 nourishment to the developing sporogenous tissue ; the transfer is 

 further provided for by the vascular system which extends upwards 

 through the centre of the " fertile frond," and even into the 

 sporangiophores of Helminthostachys. 



The effect of this theory would be to bring the Ophioglossaceae 



