204 PTERIDOPHYTA. 



cristatum. This variety has probably only become apogamous through cultiva- 

 tion. Many specimens of Isoetes lacustris, in a lake in the Vosges mountains, 

 produce in the place where the sporangia are usually found, a vegetative shoot 

 which grows into a new plant, so that the sexual generation is wanting in 

 this case. Some specimens have sporangia on some leaves, and shoots on 

 others. 



Apospory, or the formation of prothallia instead of sporangia and spores on 

 the leaves, is found in Athyrium fdix femina, var. clarissimum. In this case 

 the development of the sporangia proceeds only to a certain point, and from 

 these arrested sporangia the prothallia are produced. Normal sporangia are 

 entirely wanting in this variety, and in Aspidium angulare, var. pulcherrimum, 

 sporangia are completely wanting. Compare the Mosses (page 188). 



The Vascular Cryptogams are divided into three large classes, 

 in each of which a progressive development can be traced from the 

 isosporous to the heterosporous forms, but some of these are now 

 only known as fossils. 



Class 1. Filicinae (Ferns). The stem is small in comparison 

 with the leaves, and branches only seldom, and then by lateral 

 shoots. The leaves are scattered, large, often deeply divided, and 

 of various highly developed forms. The undeveloped leaves are 

 j-olled up in the bud, having what is termed circinate venation. 

 The sporangia are situated on the edge or on the lower side of 

 the leaves, those on which the sporangia are borne (sporophylls) 

 being often the ordinary foliage-leaves ; but in a few cases the 

 fertile differ from the barren ones (a higher stage in development). 

 The fertile leaves are not confined to definite parts of the shoot, 

 and do not limit its growth. The archesporium is most frequently 

 -unicellular. 



A. Isosporous: Sub-Class 1. Filices (True Ferns). 



B. Heterosporous : Sub-Class 2. Hydropteridese (Water 

 Ferns). 



Class 2. Equisetinae (Horsetails), in its widest meaning. 

 The leaves in this class are small in comparison with the stem. 

 They are arranged in whorls, and unite to form a sheath. The 

 sporangia are situated on specially modified, shield-like leaves, 

 which are closely packed together and form a " cone." The cone 

 is borne terminally, and limits the growth of the shoot. The 

 sporangia are developed from a large group of epidermal cells, the 

 archesporium being unicellular. The branches are arranged in 

 whorls, and develope acropetally. 



A. Isosporous: Sub-Class 1. Equisetaceae. Existing forms. 



B. Heterosporous : Sub-Class 2, Extinct forms. 



