384 PART III. THE CLASSIFICATION OF PLANTS. 



B. HETEROSPOROUS ECSPOBANGIAT^E. 



Order 3. Isoetaceae. This order includes the single genus Isoetes which 

 comprises about fifty species belonging to all parts of the globe. Some of these 

 are terrestrial (I. Duriai and Hystrlx), whilst others are either altogether 

 aquatic (e.g. I. lacustris, echinospora, etc.), or amphibious (e.g. I. velata, 

 setacea, boryana). The British species are I. lacustris, echinospora, and 

 Hystrix. 



Isoetes has, of recent years, been generally included among the Lycopodin ; 

 but it betrays a relationship to the Filicinas in so many features, such as its 

 general habit, its embryogeny, the absence of any cone-like fructification, the 

 form of its spermatozoids, that it appears to be more natural to place the plaut 

 in that group. 



SPOROPHYTE. The stem is small, unbranched, short and tuberous, with 

 either two or three longitudinal furrows which give it a lobed appearance. It is 

 closely covered with numerous, relatively long (1-12 in.), sessile leaves. From 

 the furrows of the stem there spring numerous, dichotomously branched, some- 

 what fleshy roots. 



The growth in length of the stem, which is very slow, is effected by an apical 

 growing-point consisting of several initial cells. The growing-point of the root 

 consists of small-celled meristem, and presents a similar differentiation to that 

 of the root of Dicotyledons (see pp. 145 and 154). 



The leaves are either fertile or sterile ; the fertile leaves each bear a single 

 sporangium, and are termed macrosporophylls or microsporophylls in accord- 

 ance with the nature of the sporangium which they severally bear. The order 

 of development of the leaves in each year is that first of all macrosporophylls 

 are produced, then microsporophylls, and finally a few sterile leaves in some 

 species. Hence, when the development is completed, the macrosporophylls are 

 external in the rosette, the sterile leaves (when present) internal, and the 

 microsporophylls intermediate. The sterile leaves persist during the winter, 

 and form a protection in the next spring to the young leaves developed inter- 

 nally to them at the growing-point. 



The fertile leaves, whether macro- or micro-sporophylls, consist of a broad, 

 sheathing base, with membranous margins, which bears a narrow subulate 

 lamina, flattened somewhat on the upper (ventral) surface. Close above the 

 insertion, on the upper or inner surface of the leaf-base, is a pit, the fovea, in 

 which the single sporangium is situated. In some species the margin of the 

 fovea is prolonged into a membrane, the velum, which either partially (e.g. 

 J. lacustris), or completely (terrestial species), covers the sporangium. This 

 structure appears to be homologous with the indusium present in some of the 

 leptosporangiate Ferns (see p. 391). Above the fovea, in the middle line, 

 is another smaller pit, the foveola, occupied by the somewhat swollen base of 

 a projecting flattened membranous structure, the ligule, which is developed 

 from a single superficial cell of the young foveola, and is relatively much 

 larger in the quite young leaf than in the adult. 



The sterile leaves are less highly developed than the fertile ; they are 

 smaller, especially as regards the leaf-base. In the terrestrial species they are 

 reduced to scaly cataphyllary leaves of a brown colour. The leaf grows for but 



