94 



LYCOPODIALES 



[CH. 



prominent peg-like projections, which, at first contiguous and 

 regular in contour, afterwards become less regular and separated 

 by grooves (fig. 140) and at a later stage lose their outline as the 

 bark is stretched to the tearing point (fig. 140, C). The leafless 

 branches of Lepidodendron were covered with spirally disposed 

 oval cushions less peg-like and larger than the decurrent leaf- 

 bases of Picea, which show in the upper third of their length a 

 clean-cut triangular area and swell out below into two prominent 

 cheeks separated by a median groove and tapering with 

 decreasing thickness to a pointed base, which in some forms 

 {e.g. Lepidodendron Veltheimianum, fig. 185, C, D), is prolonged 

 as a curved ridge to the summit of a lower leaf-cushion. 



ABC 



Pig. 140. Picea excelsa. Shoots of different ages showing changes in the appear- 

 ance of the leaf-cushions : a leaf attached to a cushion in fig. A. 

 (Slightly enlarged.) 



A portion of the cushion below the triangular leaf-scar often 

 shows transverse gaping cracks or depressions (fig. 185, C) such 

 as occur on a smaller scale on the older cushions of a Fir twig 

 (fig. 1 40). Secondary thickening, as in recent trees, is not confined 

 to the vascular cylinder but at an early stage, frequently before 

 there are any signs of secondary wood, the outer region of the 

 broad cortex becomes the seat of active cell-formation which 

 results in the addition of a considerable thickness to the bark. 

 At a later stage of increase in girth, the leaf-cushions are stretched 



