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When the receptacle is not fleshy, but is surrounded by an involucre, it has been called the clinanthium (the thalamus of Tournefort, as in Compositæ, or, in the language of Richard, phoranthium: Lessing calls this part the rachis. But if the receptacle is fleshy, and is not enclosed within an involucrum, as in Dorstenia and Ficus (fig. 73.), it is then called by Link hypanthodium; the same writer formerly named it amphanthium, a term now abandoned. With receptacles of this sort, which are depressed and distended branches, are not unfrequently confounded parts of a different nature, as in the Strawberry, the soft, succulent centre of which (fig. 74.) is evidently the growing point, excessively enlarged, and bearing the carpels upon its surface. See DISK, further on.
According to the different modes in which the inflorescence is arranged, it has received different names, the right application of which is of the first importance in descriptive botany. If flowers are sessile along a common axis, as in Plantago, the inflorescence is called a spike (fig. 76.); if they are pedicellate, under the same circumstances, they form a raceme, (fig. 77.), as in the Hyacinth: the raceme and the spike differ, therefore, in nothing, except that the flowers of the latter are sessile, of the former pedicellate. These are the true characters of the raceme and spike, which have been confused and misunderstood.
When the flowers of a spike are destitute of calyx and corolla, the place of which is taken by bracts, and when with such a formation the whole inflorescence falls off in a single piece, either after flowering or ripening the fruit, as in Corylus, Salix, &c., such an inflorescence is called an amentum or catkin (Catulus, Iulus, nucamentum, of old writers (fig. 82.).
If a spike consists of flowers destitute of calyx and corolla, the place of which is occupied by bracts, supported by other bracts which enclose no flowers, and when with such a formation the rachis, which is flexuose and toothed, does not fall off with the flowers, as in Grasses, each part of the inflorescence so arranged is called a spikelet or locusta.
When the flowers are closely arranged around a fleshy rachis, which is enclosed in the kind of bract called a spathe (see p. 148.), the inflorescence is termed a spadix (fig. 83.).
This is only known to exist in Aracea and Palms. It is frequently terminated, as at fig. 83., by a soft club-shaped mass of cellular substance which extends far beyond the flowers, and is itself entirely naked; this is an instance of a growing point analogous to what forms the spine of a branch, except that it is soft and blunt, instead of being hard and sharp-pointed.
The raceme has been said to differ from the spike only in its flowers being pedicellate: to this must be added, that the pedicels are all of nearly equal length; but in many plants, as Alyssum saxatile, the lower pedicels are so long that their flowers are elevated to the same level as that of the uppermost flowers; a corymb is then formed (fig. 87.). This term is frequently used in an adjective sense, to express a similar arrangement of the branches of a plant or of any other kind of inflorescence: thus, in Stevia, the branches are said to be corymbose; in others, the panicle is said to be corymbose; and so on. When corymbose branches are very loose and irregular, they have given rise to the term muscarium ; a name formerly used by Tournefort, but not now employed.
If the expansion of an apparent corymb is centrifugal, instead of centripetal; that is to say, commences at the centre, and not at the circumference, as in Dianthus Carthusianorum, we then have the fascicle (fig. 84.); a term which may not incorrectly be understood as synonymous with compound corymb. The modern corymb must not be confounded with that of Pliny, which was analogous to our capitulum.
When the pedicels all proceed from a single point, as in Astrantia, and are of equal length, or corymbose, we have an umbel (fig. 80.). If each of the pedicels bears a single flower, as in Eryngium, the umbel is said to be simple (fig. 79. a); but if they divide and bear other umbels, as in Heracleum, the umbel is called compound; and then the assemblage of umbels is called the universal umbel, while each of the secondary umbels, or the umbellules, is named a partial umbel. The peduncles which support the partial umbels are named radii. Louis Claude Richard confined the word umbel to the compound form, and named the simple umbel sertulum; but this was an unnecessary change.
Suppose the flowers of a simple umbel to be deprived of their pedicels, and to be seated on a receptacle or enlarged axis, and we have a capitulum or head. If this is flat, and surrounded by an involucre, the compound flower, as it is inaccurately called by the school of Linnæus, of Compositæ, is produced; which is sometimes named by modern botanists anthodium; it is also called cephalanthium by Richard, calathis by Mirbel, calathium by Nees von Esenbeck. The flowers or florets borne by the capitulum in its circumference are usually ligulate, and different from those produced within the circumference. Those in the former station are called florets of the ray; and those in the latter, florets of the disk.
If all the flowers are hermaphrodite in the capitulum, it is homogamous; if the outer are neuter, or female, and the inner hermaphrodite, or male, it is heterogamous; if on the same plant some capitula are composed entirely of male flowers, and others entirely of female flowers, such a plant is termed by De Candolle heterocephalous.
The glomerulus or glomus is the same to a capitulum as the compound is to the simple umbel; that is to say, it is a cluster of capitula enclosed in a common involucre, as in Echinops.
All the forms of inflorescence which have been as yet mentioned are to be considered as reductions of the spike or raceme. Those which are now to be described are decompositions, more or less irregular, of the raceme.
The first of these is the panicle and its varieties. The simple panicle differs from the raceme in bearing branches of flowers where the raceme bears single flowers, as in Poa (fig. 78.); but it often happens that the rachis itself separates into irregular branches, so that it ceases to exist as an axis, as in some Oncidiums; this is called by Willdenow a deliquescent panicle. When the panicle was very loose and diffuse, the older botanists named it a juba ; but this is obsolete. If the lower branches of a panicle are shorter than those of the middle, and the panicle itself is very compact, as in Syringa, it then receives the name of thyrsus.
Suppose the branches of a deliquescent panicle to become
short and corymbose, with a centrifugal expansion indicated by the presence of a solitary flower seated in the axils of the dichotomous ramifications, and a conception is formed of what is called a cyme. This kind of inflorescence is found in Sambucus, Viburnum, and other plants (fig. 85.).
If the cyme is reduced to a very few flowers, such a disposition has been called a verticillaster by Hoffmansegg. (Verzeichniss z. Pflanz. Cult., ii. 203.) It consitutes the normal form of inflorescence in Lamiaceæ, in which two verticillastri are situated opposite each other in the axils of opposite leaves. By Linnæus, the union of two such verticillastri was called a verticillus or whorl; and by others, with more accuracy, a verticillus spurius or false whorl. Link terms this inflorescence a thyrsula; but Hoffmansegg's name seems preferable.
The following tabular view of the differences in inflorescences will probably tend to render the above remarks more clear:
Flowers not placed on stalks,
arranged upon a lengthened axis,
which is permanent, Spike, Locusta, Spadix. which is deciduous, Catkin.
arranged upon a depressed axis, Capitulum, Glomerulus. Flowers placed on distinct stalks,
arranged upon a lengthened axis.
and of equal length, Raceme.
Inflorescence centripetal, Corymb. centrifugal, Fascicle.
Inflorescence lengthened and centripetal, Panicle.
depressed and centrifugal, Cyme, Verticillaster.
arranged upon a depressed axis, Umbel.
It occasionally happens, as in the Vine, that the rachis of