XXXV] STEPHANOSPERMUM 327 



are succeeded by an inner zone of longitudinal fibres. In the 

 structure of the palisade-layer Stephanospermum bears a close 

 resemblance to the sporocarp wall of Pilularia 1 . In the apical 

 region the shell forms a circular ridge surrounding a peri-micropylar 

 trough, a character expressed by the term 'crown-seed 2 ' employed 

 by Grand'Eury : the trough sometimes contains partially destroyed 

 tissue that may be a remnant of a sarcotesta. A Stephanospermum 

 seed, with its apical core and surrounding depression with remnants 

 of some partially destroyed tissue, may be compared with a fruit 

 of Attolia speciosa (Palmae) in which a tuft of fibrous tissue picked 

 out by decay from the mesocarp surmounts the conically pointed 

 apex of the harder interior of the fruit-wall. The base of the 

 sclerotesta is perforated by a vascular strand which expands into 

 a tracheal disc, td, fig. 494, A, at the base of the megaspore from 

 which a continuous mantle 2 3 cells broad, of short spiral and 

 scalariform tracheids (fig. 493, G), spreads over the whole of the 

 nucellus immediately below the nucellar epidermis as far as the 

 lower part of the sides of the pollen-chamber : this mantle forms 

 the floor of the large domical pollen-chamber excavated out of 

 the nucellar cone (fig. 494, A, pc). The fact that in sections of 

 older seeds the tracheal floor shows signs of splitting and dis- 

 organisation led Oliver to conclude that in the living seed the 

 tracheids underwent a gradual disintegration prior to fertilisation, 

 thus allowing the passage of the antherozoids to the egg-cells 

 (fig. 494, A, a). The presence of a continuous tracheal sheath 

 instead of separate vascular bundles is a special feature in which 

 Stephanospermum differs from Trigonocarpus and other Palaeozoic 

 seeds as well as from those of recent Cycads ; as Oliver suggests, 

 ' the apparent perfection of the vascular mantle in Stephanospermum 

 may have proved an obstacle to further development 3 ' and was 

 not retained by the more successful types. In its tracheal sheath 

 Stephanospermum resembles the seeds of Ginkgo. The nucellar 

 cone is prolonged as a beak into the micropyle formed by the 

 tubular integument. The megaspore occupies the central portion 

 of the seed and in the course of its development it compressed 

 the megasporangium (nucellus) to such an extent that little more 



1 Russow (72) Pis. in., iv. 2 (rr<?0cu/os, a crown. 



8 Oliver (04) B. p. 395. 



