ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 319 



and of their bearing on a theory of sterilisation in the sporophyte. The 

 author concludes that all that remains as the fundamental conception of 

 the sporangium in vascular plants is the spore-mother-cell or cells and the 

 tissue which covers them in. The definition of the sporangium may be 

 given thus : " Wherever we find in vascular plants a single spore-mother- 

 cell, or connected group of them, or their products, this with its protective 

 tissues constitutes the essential of an individual sporangium." From 

 the point of view of a theory of sterilisation such sporangia may be 

 regarded as islands of fertile tissue which have retained their spore- 

 producing character, while the surrounding tissues have been diverted to 

 other uses. 



The methods of variation in the number of sporangia are tabulated 

 under the heads of progressive increase and decrease ; the condition of 

 any polysporangiate sporophyte is the resultant of such modifications 

 operating during its descent. In homosporous types, which are the more 

 primitive, the larger the number of spores the better the chance of 

 survival, and hence, other things being equal, increasing numbers of 

 spores and sporangia may be anticipated ; but in heterosporous types 

 reduction in number both of spores and sporangia is frequent. Homo- 

 sporous forms are therefore regarded as in the upgrade of their evolution 

 as regards their spore-producing organs, unless there is clear evidence to 

 the contrary. The evidence of variation in numbers of sporangia in 

 the great groups of Pteridophytes leads to the result that all of them are 

 referable to modifications of a radial strobiloid type. A comparison is 

 drawn between the fertile zone in certain Bryophytes and the fertile 

 region of Lycopods. In the Bryophytes the fertile region is regarded 

 as a residuum from progressive sterilisation, and it is suggested that 

 similar causes would lead to decentralisation of the fertile tissue in the 

 primitive Pteridophytes and result in the formation of a central sterile 

 tract with an archesporium at its periphery. Such an archesporium 

 became discrete in the Lycopods ; the fertile cell-groups formed the 

 centres of projecting sporangia and were associated regularly with out- 

 growths which are the sporophylls. Whether or not this hypothesis of 

 the origin of a Lycopod strobilus approaches the truth, comparison points 

 out the genus Lycopodlum as a primitive one, characterised by more 

 definite numerical and topographical relation of the sporangia to the 

 sporophylls than in any other type of pteridophyte. 



The sporangiophore, including the soriof ferns, are placental growths 

 and not the result of metamorphosis of any parts or appendages of prior 

 existence ; it is probable that a plurality of sporangia existed on primitive 

 sporangiophores. The Lycopods, Psilotacese, Sphenophylleas, and Ophio- 

 glossacere may be arranged as illustrating the increased complexity of the 

 spore-producing parts and of the subtending sporophylls ; the factors of 

 the advance from the simple sporangium to the more complex sporangio- 

 phore are, septation, upgrowth of the placenta with vascular supply into 

 it, and branching, with apical growth also in the Ophioglossaceae. In 

 Equisetum the sporangia are regarded as directly seated on the axis and 

 therefore non-foliar ; this brings the genus into accord with the fossil 

 Calamariese. The ferns are strobiloid forms with greatly enlarged leaves. 

 The Lycopods, Psilotacese, Sphenophyllefe, Ophioglossaceas, and Filices 



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