76 THE MORPHOLOGY OF PTERIDOPHYTES 



From these were produced numerous slender forking root- 

 lets, very similar anatomically to those of Stigmaria and also 

 to those of Isoetes. Indeed, P/^wrome/a is commonly regarded 

 as a Hnk connecting the Isoetales with the Carboniferous 

 members of the Lepidodendrales. 



Isoetales 



Apart from the fossil genus Nathorstiana, the Isoetales con- 

 tain only the two living genera Isoetes and Stylites. 



The genus Isoetes is world-wide in distribution, some 

 seventy species being known, of which three occur in the 

 British flora and are commonly called 'Quillworts'. /. lacus- 

 tris and /. echinospora grow submerged in lakes or tarns, 

 while /. hystrix favours somewhat drier habitats. Most of 

 the plant is below the level of the soil, with only the distal 

 parts of the sporophylls visible. These are linear structures 

 from 8 to 20 cm long in /. lacustris, but up to 70 cm in some 

 species growing in N. America and in Brazil. They constitute 

 the only photosynthetic parts of the plant and, as in many 

 aquatic plants, they contain abundant air spaces (lacunae). 

 The expanded bases of the sporophylls are without chloro- 

 phyll and overlap one another to form a bulb-hke structure 

 which surmounts a pecuUar organ, usually referred to as a 

 'corm'. The true morphology of the corm has long been the 

 subject of controversy, for it is obscured by a remarkable 

 process of secondary growth, involving an anomalous 

 cambium. This produces small quantities of vascular tissue 

 from its inner surface and large quantities of secondary 

 cortex towards the outside. This secondary cortex dies each 

 year, along with the sporophylls and roots attached to it, and 

 it becomes sloughed off" when the new year's growth of 

 secondary cortex is produced. Vertical growth of the corm 

 is extremely slow, with the result that the body of the plant 

 is usually wider than it is high. 



Fig. 13F is a diagrammatic representation of a vertical 



