FIG. 12.--Diagram to illustrate loculicidal dehiscence. The loculaments l, split at the back, and the valves separate, bearing the septa s on their centres.
FIG. 13.--Diagram to illustrate septifragal dehiscence, in which the dehiscence takes place through the back of the loculaments l, and the valves separate from the septa s, which are left attached to the placentas in the centre.]
Dehiscent fruits, when composed of single carpels, may open by the ventral suture only, as in the paeony, hellebore, _Aquilegia_ (fig. 28) and _Caltha_; by the dorsal suture only, as in magnolias and some _Proteaceae_, or by both together, as in the pea (fig. 8) and bean; in these cases the dehiscence is _sutural_. When composed of several united carpels, two types of dehiscence occur--a longitudinal and a transverse. In the longitudinal the separation may take place by the dissepiments throughout their length, so that the fruit is resolved into its original carpels, and each valve represents a carpel, as in rhododendron, _Colchicum_, &c.; this dehiscence, in consequence of taking place through the septum, is called _septicidal_ (figs. 9, 10). The valves separate from their commissure, or central line of union, carrying the placentas with them, or they leave the latter in the centre, so as to form with the axis a column of a cylindrical, conical or prismatic shape. Dehiscence is _loculicidal_ when the union between the edges of the carpels is persistent, and they dehisce by the dorsal suture, or through the back of the loculaments, as in the lily and iris (figs. 11, 12). In these cases each valve consists of a half of each of two contiguous carpels. The placentas either remain united to the axis, or they separate from it, being attached to the septa on the valves. When the outer walls of the carpels break off from the septa, leaving them attached to the central column, the dehiscence is said to be _septifragal_ (fig. 13), and where, as in _Linum catharticum_ and _Calluna_, the splitting takes place first of all in a septicidal manner, the fruit is described as _septicidally septifragal_; while in other cases, as in thorn apple (_Datura Stramonium_), where the splitting is at first loculicidal, the dehiscence is _loculicidally septifragal_. In all those forms the separation of the valves takes place either from above downwards or from below upwards. In _Saxifraga_ a splitting for a short distance of the ventral sutures of the carpels takes place, so that a large apical pore is formed. In the fruit of Cruciferae, as wallflower (fig. 14), the valves separate from the base of the fruit, leaving a central _replum_, or frame, which supports the false septum formed by a prolongation from the parietal placentas on opposite sides of the fruit, extending between the ventral sutures of the carpels. In Orchidaceae (fig. 15) the pericarp, when ripe, separates into three valves in a loculicidal manner, but the midribs of the carpels, to which the placentas are attached, often remain adherent to the axis both at the apex and base after the valves bearing the seeds have fallen. The other type of dehiscence is transverse, or _circumscissile_, when the upper part of the united carpels falls off in the form of a lid or operculum, as in _Anagallis_ and in henbane (_Hyoscyamus_) (fig. 16).