Utricularia - bladder types
Utricularia australis is a free-floating water plant with up to 5 mm large bladders. The sides of the bladder bulge inwards indicating a vacuum was established during preparation and the bladder was in ready-to-catch condition.
Utricularia australis, immature bladder with still closed door and antennae and ruse-hairs (trichomes at the sides of the lobby) are not yet straightened out.
There are two main types of bladders in Utricularia. One represents free-floating water plants like U. australis and U. vulgaris where the door rises during bladder development and in the closed position is standing almost perpendicular to the threshold. On the right picture, the bottom edge of the door was slightly displaced when the bladder was sectioned. Species with the door perpendicular to the threshold get a larger door opening and they can thereby catch larger prey. Therefore, this bladder type is considered the most advanced.
Utricularia australis, longitudinal section through the door.
Some swamp plants like the pantropical Utricularia subulata (left and right photo) also have the door in an upright position as in the free floating species. Since the amount of water around the bladders is often modest, the guiding devices with antenna and trap-hairs are reduced and the vestibule is very short or almost none existing. When the plant grows in mud or sand with low water content, it may be an advantage that the antennas are bent towards the bladder since they may help in holding water around the door region.
The other main bladder type is characterized by the door only being raised to an 45° angel or lesser with the threshold. This type is present in most epiphytes, lithophytes and swamp plants, which in general have small bladders often only one mm large or lesser. In general, the vestibule is relatively long and tube-like such as in Utricularia reniformis (right photo). The antennas are unbranched and bent towards the bladder as it may appear from the picture to the left of a nearly mature bladder. The whole plant is densely covered with slime-glands, which together with the antennas are of importance for maintaining a water film around the bladder. This seems important when the plant is growing as an epiphyte in a moss cushion.
Utricularia lateriflora (left and right) is an endemic Australian swamp plant with a 30° inclined door and a bladder size less than one mm. The antennas are united to a single trunk-like structure bent towards the vestibule, where it may both help to maintain water and to prevent larger prey in blocking the door. There are no trap-hairs but the alluring hairs have to some extent taken over their function.
The South-African swamp plant and lithophyte Utricularia sandersonii (left and right) is likewise a species with small bladders and an inclined door. It has no antenna and the alluring hairs are also functioning as trap-hairs and they regulate the allowed size of the prey as described here. On the photo to the right, the bladder lumen is filled with some unidentified threaded mess.
H. S. Heide-Jørgensen, February 2021.