PTEROPSIDA 135 



universal in the Eusporangiatae, places them at a lower 

 level of evolution than the remaining ferns, from which it is 

 completely absent. 



The epibasal hemisphere gives rise to the shoot apex (x) 

 and the first leaf (1) (Fig. 19W), but there is no regular 

 pattern of cell divisions and the hypobasal region gives rise 

 to a poorly developed foot (f) and, somewhat later, to the 

 first root (r) (Fig. 19X). 



Chromosome counts give a haploid number n = 40 in 

 Angiopteris. 



Ophioglossales 



This group of plants, completely without any early fossil 

 record, is represented by about eighty living species, belonging 

 to three genera. Botrychhim (thirty-five species) is cosmo- 

 politan in distribution and Ophioglossum (forty-five species) 

 is nearly so, but Helminthostachys (monotypic) is restricted 

 to Indo-Malaysia and Polynesia. Two species are fairly 

 common in the British Isles, Botrychium Iwiaria, 'Moon- 

 wort' (Fig. 20A) which grows in dry grassland and on rocky 

 ledges, and Ophioglossum vulgatum, 'Adder's Tongue' (Fig. 

 20G) in damp grassland, fens and dune-slacks, while a third 

 species, O. lusitanicum, is restricted to grassy cUff tops in 

 the Channel Islands and the Scilly Isles. 



The stem, in most species, is very short and is erect, 

 except in a few epiphytic species of Ophioglossum and in 

 Helminthostachys, where it becomes a horizontal rhizome 

 as the plant grows larger. Where the stem is erect, the leaves 

 arise in a spiral sequence, but in temperate regions it is 

 normal for only one leaf to be produced each year. In 

 Helminthostachys, the leaves are borne in two ranks along 

 the rhizome ; they are large and ternately compound, but in 

 the other two genera they are usually much smaller. Those of 

 Botrychium are pinnately compound ; those of Ophioglossum 

 are simple or lobed and, unhke those of the other two 

 genera, have a reticulate venation. At the base of the petiole 



