PTERIDOPHYTA. 



219 



the submerged leaves (Fig. 217 B, s-s) and are unisexual, i.e. each 

 sorus contains micro-sporangia only, or macrosporangia. 



Azolla belongs to this order. It is a very small, floating, tropical water-plant 

 (America and East India), with horizontal, root-bearing stem. The stem branches 

 profusely by lateral buds, and bears the two rows of leaves on its dorsal side, 

 the roots on the ventral side. Each leaf is bifid, and divided into an upper dorsal, 

 and a lower ventral portion. The upper segments float on the surface of the 

 water and are arranged like tiles on a roof, each one overlapping its neighbour. 

 In each floating segment a large cavity is found, in which Anabcena is always 

 present. The lower segments are submerged. 



Order 2. Marsiliacese. The characteristic feature of this 

 order, and one not possessed by other Fern-like plants, is that the 

 sori (2-many) are enveloped in leaf- 

 segments which close round them and 

 form a " sporocarp," just in the same 

 manner as the carpels, in the Angio- 

 spermous Flowering-plants, close round 

 the ovules and form ovaries. The sori 

 contain both micro- and macro-sporangia. 

 When the spores are ripe, the sporocarp 

 opens in order to disperse the spores 

 (Fig. 220). 



The two genera (with 57 species, 

 Temperate, Tropics) are land- and marsh- 

 plants, whose dorsiventral, creeping stem 

 bears roots on the under surface, and the 

 leaves in two rows on the upper side 

 (Figs. 219, 221). The leaves *of Mar- 

 silia are compound, and divided into four 

 small leaflets springing from the apex 

 of the petiole (Fig. 219), and resemble 

 the leaves of Oxalis. In the bud the 

 leaves are circinate (Fig. 219 6), and at 

 night they exhibit the well-known sleep- 

 movements. The sporocarps are borne 

 on the petioles of the fertile leaves, near 

 their bases (Fig. 219 /) ; they are oblong 

 and resemble small beans, the outer cells 

 being hard and sclerenchymatous, while FIG. 219. Manill 



, i n i i i (natural size) : K terminal bud ; 



the inner ones are divided into a number j leayes ; f s ; porocarps . x point 

 of loculi arranged in two rows. On of branching of petiole. 



