MR. F. X. WILLIAMS ON THE GENUS DIAXTUUS. 347 



similarly as it does in Grasses, it is at this point in the vicinity of 

 the lower node of each internode that is found localized helio- 

 tropic flexion. Positive heliotropism is, however, very feebly 



expressed. It is rarely that the stems spring as absolutely simple 



from the crown of the rootstock. They may affect a simplicity 

 by a bifurcation deep in the csospitose shoots at their origin, or 

 by producing two divergent flowers at their termination. It may 

 be difficult to determine to what extent their divergence from the 

 crown of the rootstock is distinct, and how far the terminal 

 flower-cluster may be reduced to a few flowers on a stem. The 

 characteristic mode of branching is cymose, frequently in dicho- 

 tomies. Sometimes, as in D. callizonus, the simple unbranched 

 stem terminates in a solitary flower. 



Leaves. — The two leaves of each whorl are opposite. The suc- 

 cessive whorls of two leaves alternate so as to produce the 

 decussate phyllotaxis. In Syringa and Sambucus the leaves are 

 similarly arranged. It is the most frequent of alternate whorls 

 of two leaves, but in Mhamnus catharticus the two leaves of each 

 whorl are usually at a slightly different level. The mature leaf 

 is symmetrical, though in the aciculate leaves of a few species, 



ifi 



3 



as in D. barbatus, contorted at the base. The midrib is usually 

 strong, and determines, by its position in the mesophyll and itn 

 relative size to that of the leaf, the character of the laminal sur- 

 face, whether plane, carinate, or canaliculate. If it is situated 

 nearer the inferior surface of the lamina, the leaf may be botli 

 carinate and canaliculate, or it may be carinate and plane, accord- 

 ing to the thickness ; but a leaf that is canaliculate on the 

 superior surface is never plane on the inferior. The leaves are 

 connate at the base, and frequently contracted at that point, 

 resembling in this respect those of Lonicera. The form of the 

 mature leaf is linear and grass-like ; those of the barren shoots 

 are generally longer and broader than those of the flowering 

 stems. The apex is more or less acute or acuminate, sometimes 



pungent, more rarely obtuse. The margin is entire, but is not 



sharp as in that of the leaf of grasses, and is often ciliolate or 

 scabrous. The leaves of some species are of firmer texture than 

 others, occasionally almost crassulaceous. The glaucescence 

 characteristic of so many species temporarily disappears on pres- 

 sure of the leaf between the moist fingers. The cataphyllary leaf 

 is represented in the well-marked leaf-sheath. This last partakes 



2c2 



