PTERIDOPHYTA 



241 



Definite strobili were present, in Lepidodendron, at the ends of the 

 branches, in Sigillaria, in whorls along the stem. The sporophylls and 

 foliage leaves were rather similar in form. As in all lycopods, the large 

 sporangia were solitary, adaxial, and uni- 

 locular. The ligule was situated beyond 

 the sporangium. The Lepidodendrales 

 were heterosporous, the megasporangia 

 generally containing 8 to 16 megaspores. 

 In some cases trabeculae, consisting of 

 sterile plates forming incomplete parti- 

 tions, were present both in the micro- 

 sporangia and megasporangia. 



The gametophytes were developed in- 

 side the spores. The nature of the sperms 

 is unknown. The archegoniawere similar 

 to those of Selaginella. The embryo is 

 also unknown. In Lepidocarpon, a cone 

 genus, the mature megasporangium had 

 only one megaspore and, except for a nar- 

 row opening at the top, was invested by an 

 integument that arose from below. This 

 sporangium, although seed-like, was shed 

 with the sporophyll before fertilization 

 took place. 



4. Isoetales 



Isoetes is the only living genus belonging 

 to the Isoetales. It has about 60 species 

 widely distributed throughout temperate 

 regions but rare in the tropics. It grows 

 on muddy flats, in wet meadows, along 

 stream and pond margins, or submerged 

 in shallow water. A few species grow in 

 drier habitats. Fossils resembling Isoetes 

 are known from the Cretaceous and Ter- 

 tiary periods. 



Sporophyte. Superficially Isoetes is en- 

 tirely different in appearance from any 

 other living pteridophyte, resembling a 



small rush or tufted grass. Its common name is "quillwort." The stem 

 is erect, tuberous, unbranched, and very short (Fig. 198). It gives rise to 

 a crowded rosette of linear, spirally arranged leaves that are commonly 

 about 5 to 15 cm., rarely 30 cm. or more, in length. The stem is either 



Fig. 198. 

 size. 



Isoetes nuttallii, natural 



