Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/132

122 BOTANY [REPRODUCTIVE ORGANS, "VVe may recognize two distinct types of inflorescence one in which the flowers always arise as lateral shoots from a primary axis, which goes on elongating, and the lateral shoots never exceed in their development the length of the primary axis beyond their point of origin. The flowers are thus always axillary. Exceptions, such as in Cruciferous plants, are due to the non-appearance of the bracts. In the other type the primary axis terminates in a single flower, but lateral axes are given off from the axil of bracts, which again repeat the primary axis, and the development of each lateral axis is stronger than the primary axis beyond its point of origin. The flowers produced in this inflorescence are thus terminal. The first kind of inflorescence is indeterminate, indefinite, or axillary. Here the axis is either elongated, producing flower-buds as it grows, the lower expanding first ; or it is shortened and depressed, and the outer flowers expand first. The expansion of the flowers is thus centripetal, that is, from base to apex, or from circumference to centre. This kind of inflorescence is shown in fig. 156, where the leaf from which the cluster of flowers is produced, /, represents the bract or floral leaf. The rachis, or primary axis of the flower is a ; this produces small leaflets b, which bear smaller flower-leaves or bractlets, from which peduncles or secondary axes spring, each bearing single flowers. In fig. 157 the same kind of inflorescence is shown on a shortened axis, the outer flowers expanding first, and those in the centre last. Fig. 157. ?. 156. FIG. 156. Raceme of Burberry (Herberts viitgaris) produced in the axil of a leaf or bract /, which has been transformed into a spine, with two stipules s at its base; a , primary floral axis, bearing small alternate bracts 6, in the axil of which the secondary axes a", a" are produced, each terminated by a flower. The expansion of the flowers is centripetal, or from base to apex; the lower flowers have passed into the state of fruit, the middle are fully expanded, and those at the top are still in bud. Indeterminate simple inflorescence. FIG. 167. Head of flowers (capitulum) of Scabiosa atro-purpurea. The In florescence is simple .and indeterminate, and the expansion of the flowers centripetal, those at the circumference opening first. The second kind of inflorescence is determinate, de finite, or terminal. In this the axis is either elongated and ends in a solitary flower, which thus terminates the axis, and if other flowers are produced, they belong to secondary axes farther from the centre ; or the axis is shortened and flattened, producing a number of separate floral axes, the central one expanding first, while the others are developed in succession farther from the centre. The expansion of the flowers is in this case centrifugal, that is, from apex to base, or from centre to circumference. It is illustrated in fig. 158, where a representation is given of a plant of Kanunculus bulbosus ; a is the primary axis swollen at the base in a bulb-like manner b, and with roots proceeding from it. From the leaves which are radical proceeds the axis ending in a solitary terminal flower/ . About the middle of this axis there is a leaf or bract, from which a secondary floral axis a" is produced, ending in a single flower /", less advanced than the flower/ . This secondary axis bears a leaf also, from which a tertiary floral axis a" is produced, bearing an unexpended solitary 158. flower/ ". From this tertiary axis a fourth is in progress of formation. Here/ is the termination of the primary axis, and this flower expands first, while the other flowers are developed centrifugally on separate axes. A third series of inflores cences, termed mixed, may be recognized. In them the primary axis has an arrange ment belonging to the oppo site type from that on the branches, or vice versa. Ac cording to the mode and degree of development of the lateral shoots and also of the bracts, various forms of both inflorescences result. Amongst indefinite forms the simplest occurs when a lateral shoot produced in the axil of a large single foliage leaf of the plant ends in a single flower, the axis of the plant elongating beyond, as in Veronica hederifolia, Vinca minor, and Lysimachia ne- morum. The flower in this case is solitary, and the In- riant of Ranuncu us luHosus, showing florescence has been desig- determinate inflorescence. nated solitary axillary. The ordinary leaves in this case become floral leaves or bracts, by producing flower-buds in place of leaf-buds. The flowers, being all offshoots of the same axis, are said to be of the same generation or degree, and their number, like that of the leaves of this main axis, is indefinite, .varying with the vigour of the plant. Usually, however, the floral axis, arising from a more or less altered leaf or bract, instead of ending in a solitary flower, is pro longed, and bears numerous leaflets, called bracteoles or bractlets, from which smaller peduncles are produced, and those again in their turn may be branched in a similar way. Thus the flowers are arranged in groups, and frequently very complicated forms of inflorescence result. When the primary peduncle or floral axis, as in fig. 156, a, is elongated, and gives off pedicels a", of nearly equal length ending in single flowers, a raceme or cluster is pro duced, as in Currant, Hyacinth, and Barberry. If the secondary floral axes give rise to tertiary ones, the raceme is branching, and forms what is by some called a panicle, as in Yucca gloriosa ; but it is better to restrict this term to the lax inflorescence of some Grasses and Rushes. If in a raceme the lower flower-stalks are developed more strongly than the upper, and thus all the flowers are nearly on a level, a corymb is formed, which may be simple, as in fig. 159, where the primary axis a divide? into secondary axes a", a", which end in single flowers ; or branching, where the secondary axes again subdivide. If the peduncles or secondary axes are very short or awanting, so that the flowers are sessile, a spilce is pro duced, as in Plantago and Verbena officinalis (fig. 160). The spike sometimes bears unisexual flowers, usually staminiferous, the whole falling off by an articulation, as in Willow or Hazel (fig. 161), and then it is called an amentum or catkin, hence such trees are called amenti- ferous; at other times it becomes succulent, bearing numer ous flowers, surrounded by a sheathing bract or spathe, and then it constitutes a spadix, which may be simple, as in Arum maculatum (fig. 153), or branching, as in I 3 alms. A spike bearing female flowers only, and covered with scales, is either a strobilus, as in the Hop ; or a cone, as in the Fir (fig. 133). It may be mentioned here that many

do not consider the cone of Firs an axis of inflorescence,