SPH^ROSTOMA OVALE FROM PETTYCUR, FIFESHIRE. 5 



The " lagenostome " is differentiated in a manner which has so far been undescribed 

 for any ovule. We find, it is true, a central column of nucellar tissue («.), surrounded 

 by a lysigenic annular cavity — the pollen chamber — as in Lagenostoma and several 

 other genera of Palaeozoic ovules, but Sphserostoma differs in the definite manner in 

 which the roof of the pollen chamber closes the aperture made at dehiscence. As 

 far as we know, the aperture was only adventitiously blocked, or the closure was due 

 to growth phenomena in other seeds, but in Sphserostoma the pollen chamber is 

 always found tightly closed by means of what appears to be an elastically acting 

 mechanism. 



Beneath the lagenostome and plinth lies the large embryo sac (e.s.) encased in a 

 framework of eight vascular bundles (text-figs. 1 and 2, c {vide b, 1)). These spring at 

 the base from a single delicate strand which passes up the pedicel. In this particular 

 Sphxrostoma differs from the ovules included in Messrs Oliver and Salisbury's 

 groups Conostomese and Physostomese, as they are supplied by a ring of bundles. 



Accompanying the vascular bundles are large, thin-walled cells, which probably 

 formed an aqueous storage tissue. They are, however, rarely preserved in the fossil 

 except in the canopy. The wall of the ovule is strengthened by a layer of hypodermal 

 fibres which ceases at the base and is here supplemented by deeper-lying fibres closely 

 investing the single vascular strand of the pedicel, thus making the sheath complete 

 when the ovule falls (PI. II. fig. 8). 



Below the sheath in the pedicel may be seen (text-figs. 1 {ab.) and 2, d) a region 

 where rupture normally took place, suggesting that the cupule was left on the plant 

 when the ovule was shed. 



Two series of four transverse sections through single ovules enable one to state 

 definitely that the ovule is exactly circular in section in its median region, but at the 

 extreme base and in the region of the canopy the transverse section gradually assumes 

 an increasingly octagonal form {cf. text-fig. 2, c, with text-fig. 2, a, b, and d ; vide 

 also PI. II. figs. 10 and 11). 



The outermost part of the micropyle is formed by eight lobes of the canopy, which 

 bear each a large crest or lobe of the frill on the outside and a smaller one within. 

 Text-fig. 2, a, is a restoration of a transverse section through this region. It is 

 based on a section in slide 253, which is figured in part on PL I. {vide fig. 3). The 

 individual epidermal cells constituting the crest enlarge and become hexagonal in 

 section distally. They may be regarded as representing a special development of the 

 epidermal cells which are characteristically developed in all the Lagenostomales, con- 

 stituting the structure called the "blow-off" by Professor Oliver. 



Sphserostoma is most often found without the cupule, hence this organ is only 

 represented in one of the transverse sections in text-fig. 2 {vide 2, c). 



