PALEOZOIC CONE-GENUS LEPIDOSTEOBUS 



231 



The principal modifications displayed in the cone structure of Lepidostrolms may be 

 garded as falling under two heads — first, those adaptations which appear to he primarily 



concerned with the protect 



d 



pport of the sporangium, and 



ndly, those 



which facilitate the nutrition of the spores. 



Modifications for the protection and support of the individual sporangium were 

 evidently rendered highly necessary by the great bulk and the delicate structure of this 

 organ, and its " knife-edge " attachment to the sporophyll. The w^hole mass of sporangia 

 must also have been greatly in need of devices to increase its rigidity. A Tvell-developcd 

 sporophyll lamina, provided w ith a dorsal " heel " or flap, obviously affords very efficient 

 protection, both to its own sporangium and to those immediately below it. This 



protective efiect 



doubt increased by the presence of periderm, which 



& 



shown in the present paper, is frequently developed towards the outer surface of the 

 sporophyll lamina. The sporophyll pedicel is in some cases a slender stalk, triangular 

 in section, but in other cases it is provided with lateral membranous wings. These win 

 support the bulging sides of the sporangium, and this support becomes even more 

 effective when a cushion of sclerised tissue occurs on either side of the median attach- 

 ment of the sporangium {sec, PL 25. fig. 26«). These cushions are often very well 

 marked in cases where there is a ridge of sterile tissue forming the floor of the 

 sporangium, and appearing, in tangential sections, as a stalk to the sporangium. In 

 such cases the sporangium is slightly raised above the sporopliyll pedicel, and hence the 

 support given by the sclerised cushions on either side would probably become particularly 

 valuable. Another feature, which increases the rigidity of each individual sporangium, 

 is the radial plate of sterile tissue w4iich so often arises from the sporangial floor 

 {cf. PI. 24. fig. 15, &c.). 

 The resistins- power of the whole mass of sporangia is materially increased by the 



to the sporophvll pedicel, 



o 



occurrence, in some species, of a plate-like downward keel to the 



which forms a firm barrier between the adjacent sporangia lying below it, and seems, 



Williamson's words, "designed to steady the several segments of the strobilus 



(cf, PI. 22. fig. 7). 

 The large size of the sporangia must have rendered it a matter of some difficulty 



adequate supply of food and water to the developing spores 



Tb 



of a ridge or pad of sterile tissue, uniting the sporangium with the sporophyll, was 

 probably of great service in this connexion, since it increased the area of communication 

 between archesporium and sporophyll [cf. PL 25. figs. 26 & 26 a). This ridge, in one of 

 the species described in the present paper, contained numerous transfusion tracheides, by 



means of which water could be conveyed from the sporophyll 



to the develop 



spores (PL 25. fig. 27) 



A still further modification of the floor of the spo 



in the development of sterile trabeculae, or a median sterile plate, penetrating for 

 a considerable distance into the cavity. Such a structure, as Bower f has pointed out in 

 the case of L. Brownii, no doubt facilitated the nutrition of the spores. In L. Brotcnii, 

 moreover, as Zeiller t has recently shown, transfusion tracheides occur on the upper side 



"Williamson, "W. 

 Zeiller, E. ('11) 



t Bower, F. 0. ('04), p. 528. 



2m2 



