_Diadelphous_ (meaning in two brotherhoods), when united by the filaments into two sets, as in the Pea and most of its near relatives (Fig. 288), usually nine in one set, and one in the other. _Triadelphous_ (three brotherhoods), when the filaments are united in three sets or clusters, as in most species of Hypericum. _Pentadelphous_ (five brotherhoods), when in five sets, as in some species of Hypericum and in American Linden (Fig. 277, 289). _Polyadelphous_ (many or several brotherhoods) is the term generally employed when these sets are several, or even more than two, and the particular number is left unspecified. These terms all relate to the filaments. _Syngenesious_ is the term to denote that stamens have their anthers united, coalescent into a ring or tube; as in Lobelia (Fig. 285), in Violets, and in all of the great family of Compositæ. 284. =Their Number= in a flower is commonly expressed directly, but sometimes adjectively, by a series of terms which were the name of classes in the Linnæan artificial system, of which the following names, as also the preceding, are a survival:-- _Monandrous_, i. e. solitary-stamened, when the flower has only one stamen, _Diandrous_, when it has two stamens only, _Triandrous_, when it has three stamens, _Tetrandrous_, when it has four stamens, _Pentandrous_, when it has five stamens, _Hexandrous_, when with six stamens, and so on to _Polyandrous_, when it has many stamens, or more than a dozen. [Illustration: Fig. 289. One of the five stamen-clusters of the flower of American Linden, with accompanying scale. The five clusters are shown in section in the diagram of this flower, Fig. 277.] [Illustration: Fig. 290. Five syngenesious stamens of a Coreopsis. 291. Same, with tube laid open and displayed.] 285. For which terms, see the Glossary. They are all Greek numerals prefixed to _-andria_ (from the Greek), which Linnæus used for _andrœcium_, and are made into an English adjective, _-androus_. Two other terms, of same origin, designate particular cases of number (four or six) in connection with unequal length. Namely, the stamens are _Didynamous_, when, being only four, they form two pairs, one pair longer than the other, as in the Trumpet Creeper, in Gerardia (Fig. 263), etc. _Tetradynamous_, when, being only six, four of them surpass the other two, as in the Mustard-flower and all the Cruciferous family, Fig. 235. 286. =The Filament= is a kind of stalk to the anther, commonly slender or thread-like: it is to the anther nearly what the petiole is to the blade of a leaf. Therefore it is not an essential part. As a leaf may be without a stalk, so the anther may be _Sessile_, or without a filament. [Illustration: Fig. 292. Stamen of Isopyrum, with innate anther. 293. Of Tulip-tree, with adnate (and extrorse) anther. 294. Of Evening Primrose, with versatile anther.]