CLASS TETRAD YNAMIA. 
173 
to the four stamens^ have a fifth filament, which appears to be the 
rudiment of another stamen ; sometimes the irregular corolla varies 
into a regular form., with five divisions. Among those which exhib- 
it the imperfect fifth stamen, are the trumpet-flower, fox-glove, and 
Penstemon. 
In this order the personate corollas are to be found, or labiate 
flowers with closed lips. Fig. 137, c, represents a flower of this kind ; 
at is the pistil showing the capsule, or that the seeds are angio- 
spermous. It should be observed, that in this order some few flowers 
may be found with bell-form and funnel-form corollas. Plants of 
this order differ much in their natural characters, from those of the 
order Gymnospermia. None of them are used in preparations for 
food, as are the thyme and savory of the first order, but many of 
them possess pow^erful medicinal properties, as the fox-glove,* and 
the cancer-root, (Epiphegus.) They are in general a beautiful col- 
lection of plants; few flowers are more splendid than the Gerardia 
and the trumpet-flower. The Martynia is an exotic of easy cultiva- 
tion, bearing a fine blossom, while its pericarp furnishes an excel- 
lent pickle. 
As plants of this class are numerous in every part of the United 
States, you will have no difficulty in procuring them for analysis ; 
they are not usually found in blossom until the middle of summer. 
CLASS XIV. TETRADYNAMIA. 
In this class we find the cruciform plants, 
or such as have four petals in the form of a 
cross; the stamens are six, four of which 
are longer than the renaaining two. The 
cruciform tribe forms the natural order Cnt- 
cifercE^ having flowers with a calyx of four 
sepals, and a corolla of four petals; each 
petal is fastened to the receptacle or bottom 
of tlie calyx by a narrow part called a claw ; 
the whole exhibiting the form of a cross ; 
hence the term cruciform, from crux^ a cross. 
In the centre of the flower is a single pistil, 
long and cylindrical ; the stigma is oblong 
and divided into two parts, which are reflex- 
ed or bent back on each side. Each petal is 
placed between two leaves of the calyx ; this 
^.Iternate position is always seen in flowers where the number of 
petals equals the number of leaves of the calyx. The cruciform 
flowers ha,ve six stamens, two of which standing opposite to each 
other are shorter than the other four, which always stand in pairs. 
This inequality in their length determines them to be in the class 
Tetradynamia. The germ soon becomes a long pod called a silique, 
or a short thick one, called silicula : this difference in the length of 
the pods constitutes the distinction of the two orders of the class in 
which they are placed. The cabbage, mustard, radish, and stock- 
gilly-flower belong to this family. They are found, on a chemical 
analysis, to contain some sulphur. 
* See Plate vii. Pig. 6. 
Fig. 138. 
■Personate flowers— Class Tetradynamia— Describe the cruciform plants. 
15* 
