THE PTERIDOPHYTA : LYCOPSIDA, ETC. 597 



Anatomy of the Root 



The roots, like the leaves, last only for one year. Each successive row 

 formed is pushed gradually down the sides of the cleft and out over the 

 lower surface of a stem lobe, as the activity of the stelar cambium builds up 

 an increasing width of secondary cortex. The first roots of the season are 

 therefore those which lie outmost on the stem lobes, with successively 

 younger rows inwards. In the next year a new succession of roots is begun 

 and the old ones are discarded along with the moribund tissues of the old 

 stem lobes. 



It is specially noteworthy that Isoetes is the only living Vascular Cryptogam 

 which has a permanently organised root system. In all other types, except 

 perhaps in some species oiSelaginella, the roots are adventitious and irregularly 

 produced at various points. No special root-producing region exists, as 

 in the present case. 



The branching of the roots is by a true dichotomy which establishes a 

 relationship with Lycopodium and the fossil Lepidodendraceae. 



The root anatomy is very simple and singular among living plants, but it 

 is highly suggestive of the affinities of Isoetes w ith the fossil Lycopods. 



The chief feature is a very large central cavity surrounded by a cortex, 

 and containing one monarch stele, of xylem surrounded by phloem, which is 

 attached to the wall of the central cavity at one side (Fig. 607). 



Fig. 607. — Isoetes laciistris. Transverse section of root 

 showing cortex with large central cavity, at one side 

 of which the small stele is attached. 



This is almost exactly like the roots of the Lepidodendraceae, known under 

 the name oi Stigmaria (Fig. 608), which are so abundant in the Coal INIeasures. 

 This relationship is discussed in more detail at the end of the account of Isoetes. 



