APOGAMY. In some Ferns (_Pteris cretica_; _Aspidium filix mas_, var. _cristatum_; _A. falcatum_; _Todea africana_) the young plant is not developed as a consequence of fertilisation, but as a bud from the prothallium. This is known as apogamy, or loss of the power of sexual reproduction. The antheridia are generally more or less developed; archegonia are entirely wanting in _Asp. filix mas_, var. _cristatum_. This variety has probably only become apogamous through cultivation. Many specimens of _Isoëtes 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 filix 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. =Filicinæ= (=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 rolled 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. Hydropterideæ (Water Ferns). Class 2. =Equisetinæ= (=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. Equisetaceæ. Existing forms.