SPH^ROSTOMA OVALE FROM PETTYCUR, FIFESHIRE. 9 



not become at any rate normally detached. I will refer to the significance of this 

 deduction later when discussing the grounds of reference of this ovule to Heterangium 

 Grievii (cf. p. 12). 



4. Vascular Supply of the Ovule. 



It has already been demonstrated by an earlier reference to figs. 9 and 11 that a 

 single delicate strand of tracheides enters the pedicel. This gives rise to eight strands 

 which terminate behind the parenchyma abutting on the papillate epidermis near 

 the micropyle (text-figs. 1, 2, and 3, a, v.b. 2 and v.b. 3 ). The constituent tracheides 

 have their thickening disposed in a manner approximately spiral, and are of small 

 dimensions. In several cases the strand of xylem is obviously mesarch. 



At the extreme base of the embryo sac the tracheides lie almost contiguous to one 

 another, forming a sheath of which the constituent elements gradually diverge into the 

 eight strands. Very frequently one or two or even three of these strands may be 

 duplicated, i.e. respectively represented by two smaller, more or less laterally connected 

 strands. In such cases we may meet with from nine to eleven bundles, but the position 

 and size of the branching strands indicate their relationship to one another. 



5. The Nucellus and Embryo Sac. 



The nucellus exhibits features of great interest. The lower part is entirely occupied 

 by the embryo sac, which never shows much contained tissue. In all the material 

 investigated the embryo sac had already expanded until it almost abuts on the vascular 

 strands, only a few layers of much flattened cells intervening (text-figs. 1 and 2, c, 

 and PI. I. figs. 1, 5, and 6 (e.s.)). 



In the upper part the nucellus is represented by the plinth (text-fig. 1, p., and 

 PL I. and II. , figs. 1,7, and 8), and by the dome-like lagenostome already referred to 

 as consisting of a central column, surrounded by the annular pollen chamber. The 

 plinth and dome are covered with a well-characterised but apparently non-secretory 

 epidermis. 



In surface view the epidermis of the plinth is seen to consist of isodiametric 

 hexagonal cells (fig. 7), and thus offers a sharp contrast to that forming the pollen 

 chamber wall (fig. 10). The embryo sac has in all cases flattened or absorbed all the 

 cells beneath the epidermis of the plinth. It also abuts on the base of the dome, but 

 no trace of a membrane has been found beneath the excavated pollen chamber — a fact 

 which probably indicates that the archegonia were formed on that part of the prothallus 

 which forms the floor of the annular pollen chamber. In this position traces of them 

 are occasionally found. Under the central column only a thin pellicle can be demon- 

 strated, and that only occasionally. In the exact centre a thickened mass is sometimes 

 seen, but it is possibly due to the destruction of the overlying cells and not to any 

 special formation on the apex of the spore, as is rather suggested by its appearance. 



TRANS. ROY. SOC. EDIN., VOL. L. PART I. (NO. 1). 2 



