XU 
of the foramen or of the radicle, with regard to the hilum, is the 
most important feature of the embryo. When the radicle is directed 
to the extremity of the seed opposite the hilum, the embryo is said to 
be antitropal; when it is next the hilum, it is termed orthotropal ; 
when the embryo lies across the seed with its radicle towards one 
side, it is heterotropal; when so curved that both extremities are 
pointed towards the hilum, it becomes amphitropal. The cotyle- 
dons are variously arranged in the seed, being often curiously 
folded and twisted; they are sometimes leaf-like, and im some 
instances thick and fleshy. In a very few exogenous plants the 
cotyledons are wanting or are not distinguishable from the plu- 
mule. These characters of the embryo, being generally discernible 
only by the aid of a lens and distinguished with difficulty by the 
unpractised eye, have been little employed in the present work, 
The floral organs above described are all placed upon the more 
or less expanded apex of the flower-stalk, denominated the disk, 
receptacle, or thalamus. In many flowers the sepals, petals, stamens, 
and pistils are all situated immediately upon the receptacle, forming 
so many distinct concentric whorls ; but in some a different arrange- 
ment prevails,—a circumstance of which botanists have availed 
themselves in classification. Where the stamens are placed directly 
upon the disk, beneath the pistil, they are called hypogynous ; 
where they are attached to the calyx, around the pistil, perzgynous ; 
when placed apparently upon the ovary, they are said to be epagy- 
nous. It frequently happens that the filaments of the stamens in 
hypogynous flowers are attached to the petals, while in epigynous 
ones the calyx and ovary are often so united that the stamens 
appear to be in connexion with both organs. 
The manner in which the flower-buds open, technically called 
their estivation, is a feature of some mportance in distinguishing 
tribes of plants. The petals are sometimes so folded that their 
margins just meet—the sestivation is then valvate; in some flowers 
they overlap each other like tiles—it is then imbricate ; in other 
