Dehiscence of Collomia grandiflora

Abstract

THE following account of some observations of mine on the dehiscence of Collomia grandiflora may possibly prove interesting to some of your botanical readers. I can find no allusion to the singular mode in which the capsules as well as the seeds of this plant become liberated. The fruit is a three-celled capsule, and is almost wholly included within the tube of the calyx. When quite ripe it is of a pale straw colour, and becomes cartilaginous and highly polished, as does also the internal surface of the calyx tube. The latter is ribbed with fifteen prominent lines disposed in threes, each set pertaining respectively to the five sepals, and extending into their free portions. These ridges may possibly help to give direction to the capsule during its exit. Dehiscence takes place loculicidally, and the three dark-brown seeds, one in each cell, are exposed to view. It is at this stage that the phenomenon in question may be observed. The pressure exerted by the smooth sides of the somewhat obconical capsule against the equally polished surface of the calyx-tube occasions the rupture of the capsule from the base of the calyx, and its more or less rapid expulsion into the air with its three seeds. The latter, which are at this time free within the cells of the capsule, are carried to greater distances on account of the smaller amount of resistance they offer to the air by reason of their shape and weight; the action, in fact, being not altogether unlike that of the discharge of a cartridge and its contents from a rifle. The suddenness of the explosion depends very much on the state of the atmosphere at the time. On a hot day I have observed several instances of spontaneous discharges, whilst a slight touch only was necessary for the explosion of the remaining capsules whose dehiscence had already commenced. Many of the seeds were observed adhering to the upper leaves and calyx-segments, which are thickly covered with glandular hairs of a remarkably viscid nature. Contact with these moist bodies very soon induces the outgrowth of those curious and beautiful spiral hairs for which the seeds of this and a few other plants are remarkable, and thus they become doubly secured by adhesion. I have noticed in some cases when seeds adhere to the flat surface of a viscid leaf, that this outgrowth assumes a definite outline extending all round the seed in the form of a flat membranous expansion, and these, on removal, recall forcibly the appearance of ordinary winged seeds, like those of Lepigonum marginatum, for instance. Can this attachment be of any use to the seeds or to the plant itself by feeding on the nitrogenous products of their decomposition? Although I have observed a few of these attached seeds undergoing partial decay, yet, from the nature of their hard horny perisperm, it is not reasonable to suppose that it can take place to any great extent, unless the viscid secretion from the glands is able to render this substance sufficiently soluble for the purpose. If, however, a certain proportion do become sacrificed for the good of the plant, we can understand the object not only of the delicate spiral hairs for ensuring firm attachment, but also that of the explosive process, by means of which a certain number of seeds are conveyed beyond the reach of the viscid surfaces, and falling to the ground, are available for the reproduction of the plant. Saxifraga tridactylites might be mentioned as another instance of a viscid plant with the habit of retaining the seeds on its glandular parts; the much larger quantity, however, produced by this latter plant in proportion to what can be required for reproductive purposes would seem to do away with the necessity for any sudden mode of expulsion. Like most plants with sticky glandular hairs, the viscid parts of this Collomia may be seen covered with small insects in various stages of decomposition.