CH. XIII] DEFINITION OF THE SPORANGIUM 243 



If the body in question be so variable in the Class, as it is thus seen to 

 be, it is important in the first instance to have a clear definition of what is 

 meant by the word "sporangium." When all the variable features are put 

 aside there remain simply the spore-mother-cells, which in Ferns are usually 

 numerous: and there is also the protective wall, for the spore-mother-cells 

 never arise superficially. The definition of a sporangium will then be this : 

 "Wherever there is found an isolated spore-mother-cell, or a connected group 

 of them, or their products, this, together with the tissues that protect it, con- 

 stitutes the essential feature of an individual sporangium." This definition 

 gets rid of the variable accessories, however useful these may be in detailed 

 comparison, or biologically important to the plants in which they are found. 

 It fixes attention upon what is really essential, viz. the spore-mother-cells, 

 and the tissues that protect them. 



The Ontogenetic Origin of the Sporangium 

 The customary distinction of the Leptosporangiate from the Eusporan- 

 giate type of Ferns is based on the origin of their sporangia respectively 

 from one parent-cell or from a group of them. The one type is relatively 

 delicate in construction from the first; the other is relatively massive. Com- 

 parative observation shows, however, that this distinction does not depend 

 on any essential difference in kind of the two organs, but only of degree. 

 Transitional types may be found which bridge over the distinction, and 

 suggest that the one may have been derivative in Descent from the other. 

 It has already been concluded, partly from comparison but largely from 

 palaeontological evidence, that the Eusporangiate was the relatively primitive 

 and the Leptosporangiate the derivative state (p. ii 8). The series of diagrams 

 shown in Fig. 238 represents young sporangia from the several Ferns named in 

 the rubric below it. They are all based upon published drawings by authors 

 of repute. A glance at them shows that they form a series between two 

 extremes. Fig. 238, «, represents a state which may be seen in advanced 

 Polypodiaceous Ferns. It is based upon Kny's diagrams of Dryopteris 

 Filix-mas (Fig. 15, 1-3). Fig. 238,^, shows the scheme constantly presented 

 in the earliest stages of the sporangium oi Angiopteris and other Marattia- 

 ceae. It will be well to take the series in reverse of the order of lettering, 

 that is, so as to follow presumably the evolutionary sequence. The massive 

 sporangium of the Marattiaceae has its archesporium deeply sunk ; the walls 

 all cut one another at right angles, and since the outer surface is but slightly 

 convex the anticlinal walls are almost parallel, and the archesporial cell is 

 approximately cubical {g). The segmentation of the Osmundaceae is vari- 

 able, and it has been observed to be so even in sporangia of the same plant 

 in Todea barbara {e, f). In some cases the archesporium is still square- 

 based (/), and appears square also in transverse section ; it is in fact cubical, 



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