LYCOPSIDA 59 



attached annular thickenings'^^ (i.e. occasional intercon- 

 nections occur between adjacent rings), while the meta- 

 xylem tracheids are either scalariform or have circular 

 bordered pits. The phloem consists of sieve cells which are 

 elongated and pointed, with sieve areas scattered over the 

 side walls. The endodermis is clearly recognizable, in young 

 stems only, when casparian strips may be seen. In older axes, 

 however, the walls become heavily hgnified along with the 

 cells of the inner cortex and their identity becomes obscured. 

 This lignified region extends through most of the cortex in 

 some species, whose stems are consequently hard and wiry, 

 while in other species, e.g. L. squarrosum, the stem may be 

 thick and fleshy. Stomata are present in the epidermis of the 

 stem and in the leaves where, in some species, they are on both 

 surfaces (*amphistomatic') and, in others, only on the under 

 side ('hypostomatic'). The leaves of some species are arranged 

 in a whorled or a decussate manner, but in most are spirally 

 arranged. However, in these, the phyllotactic fractions are 

 said to be unlike those of other vascular plants in forming 

 part of the series f, |, xt etc.^ (whereas the normal 

 phyllotactic fractions, J, J, f, f, y\ etc., are dervied 

 from the Fibonacci series). Each leaf receives a single trace, 

 which has its origin in one of the protoxylems of the stem 

 stele and continues into the leaf as a single unbranched vein 

 composed entirely of spirally thickened tracheids. It is of 

 interest that, in L. selago, the bulbils also receive this kind of 

 vascular bundle, for this supports the view that, at this level 

 of evolution, there is no clear morphological distinction 

 between the categories 'leaf and 'stem'. This is further sup- 

 ported by the fact that leaf primordia may be transformed 

 by suitable surgical treatment into regenerative buds.^^ 



The so-called 'roots', too, show varying degrees of simi- 

 larity to stems. All, except the first root of the sporehng, are 

 adventitious and endogenous in origin, arising in the peri- 

 cycle, and they are peculiar in not bearing endogenous 

 laterals. Instead, they branch dichotomously (very regularly 



