54 STIGMARIA FICOIDES. 



Dichotomy. — An unvarying division of single organs into two more or less equal 

 structures. A growing bud has an apical cell, or cluster of cells, which divide 

 equally, giving rise to two dichotomous branches. 



Distal. — The end of an organ most remote from the organism to which it is 

 attached. 



Emergences. — Hair-like structures, which instead of being developed, like hairs, 

 entirely from a single cell of the epidermis, also originate partly from some 

 of the structures that underlie the epidermis. 



Fibees. — Prosenchymatous cells, whose walls are thickened by deposits of woody 

 substance on their inner surfaces. 



Fibeo-vascular Bondles. — Strands compounded, when perfect, of vessels, tracheids, 

 fibres and cells constituting the xylem part, and of vascular (sieve-tubes), 

 cellular and fibrous elements, the phloem or bast portion of each bundle. 



Foliar Bundles. — Fibro-vascular bundles primarily prolongations from the medul- 

 lary sheath, or, in the case of the Carboniferous Cryptogams, from the centri- 

 petal vascular cylinder, and passing outwards through the cortex to the leaves. 



Medulla oe Pith. — The central cellular axis of a stem or branch enclosed within 

 a circle of vascular bundles. 



Medullary Cavity or Canal. — A hollow space in the centre of the cellular medulla, 

 formed either by the absorption of its central cells or by their rearrangement 

 round the interior of the vascular cylinder, owing to the growth of the latter 

 proceeding more rapidly than that of the medullary parenchyma does. 



Medullaey Rays. — Radial vertical plates of cells intervening between the laminae of 

 the exogenous vascular cylinder, and connecting the medulla with the bark. 



Primary Medullary Rays. — Those first seen between the vascular bundles when 

 a vascular zone or cylinder commences its formation, and in which the con- 

 nections between the medulla and the cortex are direct and undisturbed. 



Secondary Medullary Rays. — The vessels of the first formed vascular bundles 

 which intervene between the primary medullary rays are usually few in 

 number. As the cylinder which they originate grows in thickness, these 

 bundles enlarge laterally as well as radially. The additions assume the form 

 of lamina?, which are elongated vertically, and arranged in radiating groups, 

 which latter constitute the vascularizes of transverse sections; the laminse 

 of each of these wedges are in close contact at their inner extremities, but 

 spread out at their outer ends in a fan-shaped manner. At the same time new 

 vertical cellular laminas are intercalated, though but few of these reach the 

 medulla at their inner ends. These latter cellular laininge are the secondary 

 medullary rays. 



Medullary Sheath. — Mui medullaire of Brongniart. The first formed ring of 

 vascular bundles, separating the medulla internally from the outer zone of 



