The Carboniferous Arborescent Lepidodendra. 33 



increased in number, I determined to subject them to an 

 elaborate investigation in order to observe carefully what the 

 facts were bearing upon the disputed points, and six of the 

 summer months of 1894 were perseveringly devoted to this 

 enquiry. Some preparatory observations on the general 

 structure of these plants will help students to understand 

 what these points are. In the youngest Lepidodendroid twigs 

 the conspicuous central tissue is a small vascular bundle 

 known as the Primary Xylem strand. It extends, under 

 varied modifications of form and size, from near the apex of 

 the youngest twig to the base of the oldest stem. In its down- 

 ward course it gives off a large number of small vascular 

 bundles, known as leaf-traces, each one of which passes 

 outwards to a leaf, supplying it with its vascular tissues. 

 In many cases we discover a few cells in the centre of its 

 component tracheids, which, on passing downwards 

 towards the lower members of the tree, enlarge into a more 

 or less conspicuous medulla. In a few cases the smaller 

 shoots exhibit no traces of these cells, which are only 

 discoverable in branches of somewhat larger size ; but in 

 all, the larger the twig, the larger also is the central cellular 

 tissue in varying degrees and in different types. This 

 is a true medulla, which generally exhibits its maximum 

 diameter only at the base of the oldest stems. Externally 

 to this Primary Xylem system, and in the closest con- 

 tact with it, a second vascular zone, known as the 

 Secondary Xylem, is developed, much like the wood of 

 ordinary trees, from a cambium layer investing its periphery. 

 This second tissue system is composed of vertically pro- 

 longed, radiating, vascular laminae, which are separated by 

 intervening medullary rays. This Secondary zone and the 

 Primary Xylem enclosed within it form what is known by 

 the name of the stele; and, as in these Carboniferous Lyco- 

 pods we have but one such concentric combination of tissues, 

 these stems are said to be monostelic. 



