12 PROF. MARGARET J. BENSON ON 



The tongue -shaped cells already described, which otherwise seem so enigmatic, 

 receive their interpretation if they were the source of the water required for the closure 

 of the pollen chamber. 



Summary of Stages. 



1. Growth in length and special thickening of the roof-cells of the pollen chamber. 

 Concomitant lysigenic degeneration of the subjacent tissue leading to the excavation 

 of the pollen chamber. 



2. Circumscissile dehiscence and consequent formation of a stomium by the upward 

 movement of the free margin of the roof. 



3. The retention of the pollen grains on the downward curvature of the roof, 

 which thus returns again to its original position and closes the pollen chamber. 



8. Diagnosis of Sphxrostoma, n.g. 



The ovule resembles those of the Lagenostoma series in its general organisation, 

 e.g. the nucellus is surrounded by an inner integument of radial symmetry and appar- 

 ently multiple origin, and also by an outer integument or cupule. The inner integu- 

 ment is only free around the pollen chamber, but the cupule is free from the base 

 upwards. Both integuments have a multiple vascular supply — the strands of the inner 

 integument taking their origin from the single central bundle of the pedicel. Sphwro- 

 stoma is distinguished from the ovules of the Lagenostoma series by the following 

 characters : — 



1. The whorl of epidermal crests around the micropyle. 



2. The nearly hemispherical form of the lagenostome which rises from a somewhat 

 flat plinth. 



3. A pollen chamber relatively wider than that of Lagenostoma. 



4. The persistence of the epidermis over the central column of the lagenostome. 



5. A skeletal structure of less robust charater, with an indication of aqueous tissue 

 in the region of the canopy. 



Dimensions. — The ovule is a little under 3 '5 mm. in length without the cupule 

 and 2*2 mm. at its widest part. 



Horizon and Locality. — Sphterostoma ovale has so far only been recorded from 

 the Calciferous Sandstone Series of the Lower Carboniferous rocks of Pettycur, 

 Kifeshire, Scotland. 



[V. The Grounds for the Provisional Reference of the Ovule Sph^erostoma 



OVALE TO HETERANGIUM GrIEVII. 



As already mentioned, this ovule has long been surmised to be the mega- 

 sporangial apparatus of LLeterangium Grievii. The association between the two is 

 of ;i very constant and striking nature. For example, there are some score of plants 



