CARBONIFEROUS LYCOPODS AND SPHEXOPHYLLS. 29 



The sporangia are globular, shortly stalked, and spring from 

 the stem at the insertion of the sporophyll. The microsporangia 

 contain numerous spores, the macrosporangia generally four 

 macrospores. The spores, both micro and macro, have three 

 ridges, which radiate from the centre. Their surface is variously 

 ornamented. In Selaginella caulescens the macrospores have an 

 equatorial, zone- like wing. 



Isoetes, Linn. 



The stem is very short, unbranched, and increases in thickness 

 from a layer of meristem which surrounds its vascular body and 

 produces new layers of parenchyma on the outside. This increase 

 in the size of the stem usually takes place at two or three points, 

 according to the species, so that in section the stem exhibits 

 two or three projecting masses of tissue. 



The leaves are long and sub-triangular, convex behind and con- 

 cave in front, and consist of two parts — a basal portion or sheath, 

 and an upright part or lamina. They are developed in a close 

 rosette, and each leaf contains four air-cavities divided by septa. 



The sporangia, which are attached by a narrow band, are 

 placed in a hollow at the base of the leaf (the fovea). The 

 hollow is covered more or less by the velum or indusium. Above 

 the fovea is another small pit-like hollow, the foveola, from the 

 base of which springs the ligule, a triangular scale-like outgrowth. 



The sporangia are imperfectly divided into compartments by 

 bands of tissue which stretch across from the ventral to the 

 dorsal side. The lower leaves bear the macrosporangia, the 

 upper the microsporangia. 



The macrospores are large and globular, and bear three 

 prominent ridges, which radiate from the apex to the middle ; 

 their outer surface is generally more or less granulated. The 

 microspores are very small and trigonous, with a ventral rib. 



Some members of the genus are aquatic or sub-aquatic, and a 

 few are terrestrial. 



CLASSIFICATION OF CARBONIFEROUS LYCOPODS 

 AND SPHENOPHYLLS. 



So many points still remain unknown, or are only insufficiently 

 known, in connection with the structure and fructification of 



