82 BOTANICAL GAZETTE [august 



of a normal sporangium in place upon a detached sporophyll is 

 rare, while, on the other hand, the sporangia of Lepidostrohus held 

 so tenaciously to the sporophyll and the latter to the cone axis that 

 both among petrifactions and impressions we find long series of 

 apparently mature sporangia in place. It is conceivable that the 

 very mechanical excellence of this attachment might thus prove 

 the undoing of a type in close competition with another with 

 no such restriction upon the dissemination of its spores. This 

 mechanical weakness in Cantheliophorus is well compensated for 

 in protecting structures at the time when otherwise it might prove 

 fatal. The most conspicuous of these is the guard and the blade. 

 The latter protects in one of two ways. In species hke C. novacu- 

 latus where it is flexible it is closely appressed, and in this way forms 

 a dense, overlapping, protecting vestiture for the entire cone; while 

 in other species, well illustrated by C. ensifer, it stands out formi- 

 dably, rigid and sharp-pointed, like a bayonet. The simple but 

 very efhcient expedient by which this rigidity is secured is interest- 

 ing from a mechanical point of view. It is secured by backward 

 folding along the stomatiferous furrows near the midrib until the 

 moment of inertia about the horizontal neutral axis of the cross- 

 section approaches that about the vertical neutral axis ; terms which 

 express resistance to bending in a cantilever channel, or angle-beam 

 of equal legs such as this. The sporangium is further protected on 

 one side perfectly by the sporangiophore and on the other, in some 

 species at least partially, by the crest and keel of subjacent and 

 superjacent sporophylls respectively. Very short, lateral, flangelike 

 wings on the pedicel serve as rests for the sporangia. The large 

 radial sporangiophores with their tenacious hold upon both pedicel 

 and axis, further strengthened by the brace and the guard, probably 

 gave to the cone as a whole great stability during the period of the 

 development of the sporangia. 



In strobiloid types the increase in the volume of the sporangium 

 by radial extension may be regarded as an eflicient simple struc- 

 tural modification for increasing spore production, because this 

 result may thus be accomplished without at the same time disturb- 

 ing the cone by the multiplication of parts or by the modification 

 and the increase in the size of the necessary protective structures. 



