PTEKIDOPHYTA. 238 



axis in the elongation of the suspensor, but in Selaginella the em- 

 bryo is situated transversely to it. 



SelagineUa (300-400 species), is essentially tropical, only one 

 species living in the North (S. spinulona) , but others grow in 

 Central and South Europe. 



Order 3. Lepidodendraceae are extinct, tree-like Lycopods, which are 

 found especially in the Lower and Middle Carboniferous. Vegetatively they are 

 most nearly related to Lycopodium, but the stem attained much larger propor- 

 tions (about eleven metres in height and one metre in thickness), and had a 

 cambium by which it increased in thickness. It was regularly dichotomous, 

 and closely studded with spirally-placed leaves, which left behind them peculiar 

 rhombic scars. The large cones resemble Pine-cones, and bore sporangia 

 much larger than any which are now produced (the male ones as much as 

 2 cm.'s in length). The macrosporangia were situated at the base, and the 

 microsporangia at the apex. 



Order 4. Sigillariaceae. These are, presumably, another group of extinct 

 tree-like Lycopods (especially in the Middle Carboniferous). The name has been 

 derived from the seal-like scars, which the fallen leaves have left behind in 

 longitudinal rows on the grooved stem. The rhizomes of these plants were 

 formerly termed Stitjinariu, and placed in a separate genus. 



Order 5. Sphenophyllaceae form an entirely extinct group. They do not 

 definitely belong to any of the three large classes of Vascular Cryptogams, but 

 it is perhaps best to place them in juxtaposition to these. They were herbaceous 

 plants with verticillate, wedge-shaped leaves, with nerves branching dicho- 

 tomously into equally strong branches. Micro- and macro-sporangia were 

 formed in the same cone ; and were situated in the axils of the leaves, as in the 

 Lycopods. 



