338 HOEMEISTER, ON 



and will be found in accordance with those reported by 

 Mold. I am indebted to the kindness of Mettenius,* 

 Alexander Braun, and Gustav Reichenbach,f for the abun- 

 dant materials upon which my observations are founded. 



The large spores of Isoetes lacustris are at first tetra- 

 hedral with a convex basal surface. As they ripen, their 

 remaining surfaces also become gradually arched, and 

 assume almost a spherical form. The delicate primary cell- 

 membrane is clothed with a thick exosporium, which, 

 when cut through, exhibits three principal layers. The 

 innermost is a glassy membrane, of a brown colour and 

 moderate thickness, upon which ridges of different lengths, 

 and converging towards the pole of the spore, are seated. 

 Three of the longer and more prominent of such ridges, 

 .answering; to the eclo'es of contact of the spore with its 

 three sister-spores, unite at the apex at angles of 120°. 

 They reach to the equator of the cell, and there intersect a 

 somewhat less prominent annular ridge, which surrounds 

 the spore. This innermost layer of the exosporium is 

 succeeded by a thinner layer, of a granular consistence 

 and yellowish colour, over which the thick outermost 

 covering, consisting of a transparent gelatinous mass, is 

 spread. Like the former one, it covers all the rids-e-like 

 protuberances of the innermost glassy layer of the exos- 

 porium, and it is especially fully developed over the four 

 principal ridges (PI. XLVI, fig. 1). 



The matter composing the exosporium behaves towards 

 reagents like the exine of pollen-grains. Sulphuric acid 

 imparts a reddish colour to the inner layers, which are soft- 

 ened by boiling in alkaline lej^s. The gelatinous layer is 

 rapidly destroyed by mineral acids and caustic alkalis. As 

 Roper | has observed, the exosporium does not contain 

 carbonate of lime, although Schleiden§ suspected its pre- 

 sence from the appearance of the dry spores. The contents 

 of the ripe spore in its optical and chemical characters 



* I received living specimens of Isoetes lacustris from the lake in the Black 

 Forest, the same habitat which afforded the materials for the observations of 

 Bischoff, Molil, Braun, and Mettenius. 



f Dried specimens of species of the Mediterranean Flora. 



% 'Zur Flora Mecklenburg's/ vol. i, p. 125. 



§ ' Grundzuge,' 2nd edit., vol. ii, p. 84 



