BOTANY. gz 
OrvER 39. Dioscorrace«, the Yam Tribe. Flowers unisexual. Perianth 
six-divided, adherent. 4%. Stamens six, inserted into the base of the perianth; 
anthers introrse, with longitudinal dehiscence. %. Ovary inferior, three- 
celled ; ovules one to two, anatropal; style bifid; 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 hilum, lying in a large cavity of 
cartilaginous albumen. ‘Twining shrubs, with large epigeal or hypogeal 
tubers, alternate, sometimes opposite, reticulated leaves, and small, spiked. 
bracteated flowers. Natives chiefly of tropical countries ; a few only found in 
temperate regions. There are six genera according to Lindley, 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 Evogene, Juss.and D.C. Acramphibrya, Endl. 
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. T'he stem is more or less conical, and exhibits wood 
and true bark. The wood is exogenous, 1. 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. T'he epidermis 
is furnished with stomata. T'he leaves are reticulated, usually articulated to 
the stem. The flowers are formed upon a quinary or quaternary type, and 
have stamens and pistils. Zhe 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. T'he embryo has two or more 
opposite cotyledons, and is exorhizal in germination. 
Sub-class 1. Monochlamydec. 
Corolla wanting ; a calyx or a simple perianth present; flowers sometimes 
achlamydeous. This sub-class includes the Apetalous orders of Jussieu, ani 
many of his Diclinous irregular orders. It corresponds to the Apetale and 
Gymnosperme of Endlicher. 
Section A. Gymnosperme. 
Monochlamydeous or Achlamydeous plants, with an exogenous structure 
as regards their stems and organs of vegetation, but differing from Exogens, 
in having naked ovules, which are fertilized by the direct application of 
the pollen to the foramen, without the intervention of stigma, style, and 
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