216 THE POTOMAC OR YOUNGER MESOZOIO FLORA. 



short, the jointing most conspicuous on those parts which retain the cortical 

 layer and epidei-mis, especially distinct in the smallest ultimate branches, 

 the jointing on the larger decorticated stems often not visible; woody axis 

 proportionally very small, apparently surrounded originally by a thick suc- 

 culent cortical layer, which was covered by a firm, parchment-like, very 

 durable epidermis ; this latter marked by very fine longitudinal lines 

 formed of rows of small tubercles or dot-like elevations; in the smallest or 

 ultimate branches the woody matter seems to have been almost or wholly 

 wanting, the material composing the branch being succulent and covered 

 with a firm epidermis; leaves in whorls of threes, leaves of successive 

 whorls alternating; on the youngest branches the leaves are often retained 

 uninjured in the form of minute, closely appressed scales, mostly broadly 

 triangular in form, acute, often acuminate, sometimes mucronate, united at 

 base ; on the older branches often convex in form, and under a lens show- 

 ing towards their tips a prominent line forming a sort of keel ; on these 

 branches, when deprived of epidermis, the leaves leave scars, rhomboidal 

 in shape, and slightly elongated in the direction of the length of the stems. 

 The shape of the older leaves varies more than that of the j^ounger ones, 

 being broadly triangular, with very short acute tips or nuicronate, some- 

 times dome-sliaped to broadly conical, with tips prolonged to various 

 lengths, often acuminate and sometimes mucronate ; the ultimate succulent 

 brandies, seeming to have performed the woi'k of leaves and always short, 

 go off from the joints in the axes of the scale-like leaves, generally curve 

 upwards, and apparently lie in the same plane. The subordinate twigs 

 always end with a branch like those sent off lower down ; ultimate 

 branches in length, stoutness, number of branches on the penultimate twigs, 

 and degree of crowding show considerable variation ; in some cases the 

 ultimate branches are very closely placed, three or four on a side, with a 

 terminal one short and stout, in other cases they are remotel}^ placed, stoiit 

 and long; again, the plants have sometimes on the penultimate twigs a 

 few remote, very short branches, hardly more than buds ; buds sometimes 

 found on short twigs, arranged as are the branches, forming' club-shaped 

 growths, in which a number of internodes with partially formed leaves are 

 shown. 



