BOTANY. 87 



Order 39. Dioscokeacete, the Yam Tribe. Flowers unisexual, rerianth 

 six-divided, adherent, j. Stamens six, inserted into the base of the perianth; 

 anthers introrse, Avith longitudinal dehiscence. ? . Ovary inferior, three- 

 celled ; ovules one to two, anatropal ; style l)ifid ; stigmas undivided. Fruit a 

 compressed trilocular capsule ; "with two cells, sometimes abortive, occasionally 

 fleshy and indehiscent. Seeds compressed, -winged or -wingless, in the succu- 

 lent fruit ovate ; embryo small, near the hiluni, lying in a large cavity of 

 cartilaginous albumen. Twining shrubs, -with large epigeal or hypogciil 

 tubers, alternate, sometimes opposite, reticulated leaves, and small, spiked, 

 bracteated flowers. Natives chiefly of tropical countries ; a l\?w only found in 

 temperate regions. There are six genera according to Lindlcy, and 110 species. 

 Examples : Dioscorea, Tamus, Elephantopus. 



The Yam, a tropical substitute for the potatoe, is the tuber of several 

 species of Dioscorea. 



Class 3. Dicotyledones and ExogencB, Juss. and D.C. Acrampliibrya, End). 



This is the largest class in the vegetable kingdom. The plants included 

 under it have a cellular' and vascular system, the latter consisting partly of 

 elastic spiral vessels. The stem is more or less conical, and exhibits -wood 

 and true bark. Tlie wood is exogenous, i. e, increases by additions at the 

 periphery, -the hardest part being internal. It is arranged in concentric 

 circles. Pith exists in the centre, and from it diverge medullary rays. The 

 bark is separable, and increases by additions on the inside. The epidermis 

 is furnished -with stomata. The leaves are reticulated, usually articulated to 

 the stem. The flowers are formed upon a quinary or quaternary type, and 

 have stamens and pistils. The ovules are either inclosed in a pericarp, and 

 fertilized by the application of the pollen to the stigma, or they are naked, 

 and fertilized by the direct action of the pollen. The embryo has two or more 

 opposite cotyledons, and is exorhizal in germination. 



Sub-class 1. MonochlamydefC. 



Corolla wanting ; a calyx or a simple perianth present ; flowers sometimes 

 achlamydeous. This sub-class includes the Apetalous orders of Jussieu, and 

 many of his Diclinous irregular orders. It corresponds to the Apetalfe and 

 Gymnospermae of Endlicher. 



Section A. GymnospermcB. 



Monochlamydeous or Achlamydeous plants, -with an exogenous structure 

 as regards their stems and organs of vegetation, but diftering from Exogens, 

 in having naked ovules, which arc fertilized by the direct application ot 

 the pollen to the foramen, without the intervention of stigma, style, and 



87 



