278 The Botanical Gazette. [October, 



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California', while the third one, P. Sabinei R. Brown, is a high 

 arctic type. Specimens of this were collected at Cape York 

 in northwestern Greenland by the Swedish paleontologist 

 Nathorst, who has kindly furnished me with several finely 

 preserved individuals. 



The leaf structure shows in these species a rather uniform 

 aspect, which strikingly indicates their occurrence in wet 

 meadows, although they live under such widely different 

 climatological conditions. Very characteristic are the large 

 lacunes in the mesophyll, the merely two groups of bulliform- 

 cells in the carene, and the proportionally weak development 

 of the stereome. 



Pleuropogon refr actum Gr. — The epidermis cells are rectan- 

 gular with nearly straight and rather thin walls and there are 



on both faces of the leaf numerous conical warts. The superior 

 face shows also several thick-walled and sharp-pointed expan- 

 sions, which are directed upwards and which form long lines 

 above the stereome-bundles (plate XXIV, fig. 9). Stomates 

 are present on both faces of the leaf and are partly surmounted 

 by conical warts. The bulliform cells form two groups, one 

 at each side of the midrib ; they are large and their exterior 

 walls are entirely smooth in contrast to the other cells of the 

 epidermis (plate XXIV, fig. 8). 



The mestome-bundles represent two degrees of develop- 

 ment; the median one (plate XXIII, fig. 6) is the largest in 

 the whole blade and forms a slightly prominent carene. It is 

 surrounded by a thin-walled parenchyma-sheath of which a 

 part contains chlorophyll. Inside this, the proper sheath, 

 is also to be observed a true mestome-sheath, the cells of 



which are somewhat thickened. The leptome is separated 



from the hadrome by a single stratum of thick-walled mes- 

 tome-parenchyma, and there is a small group of stereome 

 above and below the entire bundle. As to the corresponding 

 mestome-bundles in the lateral parts of the blade, these differ 

 from the median one merely by having a few cells of uncolored 

 parenchyma on both faces, so that the stereome is not here in 

 contact with the parenchyma-sheath. A small mestome- 

 bundle, representing the second degree, has been figured on 

 plate XXin, fig. 7, where is a completely uncolored paren- 

 chyma-sheath with a thin-walled mestome-sheath inside this. 

 The leptome and hadrome are not so strongly developed as in 

 the preceding and they are not separated from each other by 



