118 LYCOPODIALES [CH. 



secondary tissue (phelloderm, pd) passing towards the inner 

 boundary of this region into the primary outer cortex ^r (fig. 151, 

 A and C). The radially disposed elements which make up the 

 bulk of the phelloderm are associated with concentric rows of 

 secretory strands, represented by tangentially arranged dots in 

 fig. 151, A: on the outer edge of the phelloderm a few patches of 

 primary cortex are still preserved, as at c, fig. A. One of these 

 is shown on a larger scale in tig. B ; at wj the phelloderm is 

 interrupted by a gap beyond which the cells have thinner walls 

 and show signs of recent division ; this is probably the position 

 of the phellogen. The tissue b, fig. 151, B, consists of secondary 

 cortex succeeded beyond d by the parenchymatous tissue of the 

 leaf-cushion, in which the remains of a ligule, I, are seen in the 

 ligular pit. This section corresponds in position to a line 

 drawn across fig. 150 at the level of b. In this specimen we 

 have two kinds of secondary cortical tissue : that formed external 

 to the phellogen, from m to d in fig. 151, B, is less in amount 

 than that produced internal to the phellogen. We cannot make 

 any satisfactory statement as to the nature of this secondary 

 tissue, whether or not any of it agreed in composition with the 

 cork which is usually formed external to the phellogen in recent 

 plants. As the stem of a Lepidodendron grew in girth the leaf- 

 cushions became separated by intervening depressions composed 

 of the secondary cortex formed external to the phellogen, but at 

 a later stage the cushions were thrown off, leaving the outer 

 edge of the phelloderm as the superficial tissue. This exposed 

 tissue became fissured as growth and consequent stretching 

 continued, producing the appearance seen on the surface of 

 the still older stem represented in fig. 153. 



The inner edge of the phelloderm seen at e in fig. 151, C, 

 passes suddenly into the inner primary region of the outer cortex 

 Cfig. 151, A and C, g) which comprises two types of parenchy- 

 matous tissue, patches of isodiametric cells, g, g, alternating 

 with radially arranged areas consisting of tangentially elongated 

 elements (fig. C,//; fig. D) which extend as wedges into the 

 phelloderm. 



The longitudinal section represented in fig. 152, B, shows an 

 equal bifurcation of a stem in which no secondary xylem is 



