Byblis – Rainbow plant
Byblis (Rainbow plant) is a small genus with 2 (possibly 3) species in the family Byblidaceae. The genus was first described in 1848, but was only recognized as carnivorous in 1875 by Darwin. The two recognized species are Byblis liniflora (Figure left) and B. gigantea, which differ from each other especially with respect to distribution, size and life span. The first grows in coastal swampy areas of tropical northern Australia. It is usually annual and rarely over 30 cm high with approx. 1 cm large flowers (top).
Byblis gigantea (right) grows twice as tall, is perennial and woody in lower stem parts. It is found on moist-sandy soil in Western Australia in the area around Perth. The flowers are approx. 2 cm in diameter. It is now and then exposed to steppe fires, which the rhizome usually survives. The fires seem to promote seed germination.
The South American genus Roridula was previously considered carnivorous belonging to the family Byblidaceae. It is a shrub with sticky glandular hairs that trap insects, but the secretion is resinous and Roridula cannot digest the catch. It is now relegated to its own family Roridulaceae.
Trap construction and function
The leaves are 10-30 cm long and thread-shaped with a triangular cross-section, and they are densely studded with glandular hairs, where the secreted mucus refracts the light in the colors of the rainbow in sunshine. The glandular hairs also cover stems, flower stalks and sepals. The leaves function both as a photosynthetic organ and as a glue stick for catching insects. The long-stalked glandular hairs secrete a sticky muco-polysaccharide mucus that primarily captures flying insects. In addition to Byblis, two other genera have traps that act as glue sticks, Triphyophyllum and Drosophyllum. As described on this website for the latter genus, the wind sometimes causes the leaves to clash so that the mucus is drawn out into threads that can form and act as a spider’s web.
The mucilaginous hairs are derived from a single epidermal cell that has divided into an elongated basal cell, a short endodermoid stalk cell that corresponds to the endodermoid cell layer of Drosera and Drosophyllum, and a gland head consisting of 16-32 radially arranged gland cells.
The ‘spider web’ increases the effectiveness against randomly flying insects, and this can perhaps compensate for the fact that the glue stick has only to a small extent developed lures in the form of e.g. scent and nectar, although it cannot be ruled out that the sun glitter in the slime drops can be attractive to certain insects. Also, like Drosophyllum, Byblis does not follow the general principles of carnivorous plants regarding the separation of flowers and traps, so with the flowers hidden between the leaves, potential pollinators run a greater risk of ending up as prey.
Above is a section through a mucus-secreting gland cell of Byblis linifolia laid perpendicurlarly to the radial cell walls and photographed in an electron microscope at 5800x. D, two mucus-producing dictyosomes (golgi bodies). F, mucus stored in porous cell wall. M, mitochondrion. N, large cell nucleus. S, starch granule in plastid. W, middle lamella of the cell wall.
Below are mucus-producing gland cells of Byblis linifolia photographed at 29,000x.
D, jar-shaped dictyosome. V, vesicle.
D, jar-shaped dictyosome in cross section. M, mitochondrion. P, plastid.
V, vesicle ready for detachment from the dictyosome.
Arrow, vesicle fuses with the cell membrane. Star, newly fused vesicle.
The muco-polysaccharide mucus is produced in dictyosomes. It is an organelle that consists of tightly stacked, compressed membrane sacs, which in Byblis form a jar. Inside the jar, a number of small membrane-clad blisters can be seen, some of which are placed in a row. The vesicles merge into the innermost and thus youngest membranous sac in the dictyosome, and the vesicles contain precursors to the mucus, which is then completed in the membranous sac and finally untied as a mucus-filled vesicle from the edges of the older membranous sacs. Cytoplasmic currents then carry them towards the cell membrane, with which they fuse, thus emptying the mucus into the porous cell wall. It is an energy-demanding process, and the cells therefore contain many mitochondria. As the mucus pressure increases in the cell wall, the outermost membrane, the cuticle, on the cell cracks or bursts, and the mucus flows out. As the mucus droplet is several times larger than the gland head, a significant supply of water takes place via the stem during secretion.
The digestive glands
The leaves are also equipped with digestive glands that sit in a row (are placed in a row), partially immerged in furrows on the leaf surface (photo above and at left). They are built on the same three-part plan as the mucus-secreting gland hairs, but the basal cell (possibly two) is very short, the 8 gland cells are transfer cells equipped with labyrinth walls that enlarge the surface of the cell membrane by up to 20 times (photo below).
At left, immerged digestive glands and prominent stoma in Byblis linifolia.
Transfer cells generally appear where transport bottlenecks arise and/or where there is a need for temporary storage of secretion products. At right, the irregular labyrinth wall of a digestive gland is seen. K, swelling of cuticle. M, mitochondrion.
Unlike the mucous glands, the digestive glands are dry as long as there is no prey in the trap. Uric acid secreted from the prey as well as a number of other nitrogenous compounds are able to stimulate the gland so that the enzymes are washed out from vacuoles and cell walls by help of a flow of water through the gland in the same manner as described for Drosophyllum.
In Byblis, there is a swelling and bursting of the cuticle over the stimulated gland cells (photo left and above), so that the mucus can flow freely. As with all other carnivorous plants, the leaf is also used for photosynthesis. Therefore, the glue sticks are provided with stomata. As the digestive fluid flows over the leaf lamina, some stomata will be put out of action. At Byblis, the problem is counteracted by the fact that the digestive glands are immersed in furrows, while the stomata are accentuated.
H. S. Heide-Jørgensen, November 2020/Translated January 2025.
Return to Carniverous plants