406 PART III. THE CLASSIFICATION OF PLANTS. 



Order 5. Schizaeaceae, including the genera Schizaea, Aneimia, Mohria and 

 Lygodium, with a projecting apical annulus to the almost sessile sporangium, 

 occur only in the tropics. Lygodium is the most remarkable genus ; its pinnate 

 leaves grow to a great length, and twine round supports by means of their 

 midribs : it alone has an indusium, and the sorus is usually unisporangiate. 



Order 6. Osmundaceae. The shortly-stalked sporangia have a rudimentary 

 annulus consisting of a group of cells (Fig. 261 B r) just below the apex ; they 

 burst open by a longitudinal slit on the side opposite to this. 



Osmunda regalis, the Fern-Koyal, is a not very common but well-known Fern. 

 Only the upper pinnae of the leaves are fertile, and develope little or no meso- 

 phyll ; the sori are marginal, and consist of a great number of sporangia ; they 

 have no indusium (Fig. 261 A s). The only other genus is Todea, belonging 

 mainly to Australasia. 



D. HETEROSPOROUS LEPTOSPORANGIAT^E. 

 (Hydropteridese or Rhizocarpae.) 



This group includes the four genera, Salvinia, Azolla, Marsilea, 

 Pilularia; of these the two former constitute the order Salviniaceae, 

 the two latter the order Marsileaceaa. They are all more or less 

 aquatic in habit, Salyinia and Azolla being free-floating fugacious 

 plants, whilst Marsilea and Pilularia are perennials growing in 

 bogs and marshes. 



SPOROPHYTE. The stem is a horizontal dorsiventral rhizome. It 

 generally bears foliage-leaves in alternating longitudinal rows 

 (four rows in Salvinia; two rows in the other genera) on the 

 dorsal (superior) surface ; and roots in one (Marsileacea?) or two 

 (Azolla) longitudinal rows on the ventral (inferior) surface. In 

 Salvinia, however, there are no roots, but the stem bears in place 

 of them two rows of submerged leaves on its ventral surface. The 

 lateral branches, sometimes very numerous, are borne on the 

 flanks. 



In the Salviniaceae the apical growing-point of the stem has a 

 two-sided apical cell, situated in the vertical plane, from which 

 lateral segments are cut off alternately right and left : each 

 segment is divided by a horizontal wall into a dorsal and a ventral 

 half, and each of these is again divided by a transverse wall so that 

 each segment gives rise to four cells: thus the growing-point 

 -consists fundamentally of eight longitudinal rows of cells, four 

 belonging to the dorsal, and four to the ventral region, of which 

 the two uppermost may be designated the dorsal rows, the two 

 next the dorso-lateral rows, the two next the ventro-lateral rows, 



