UTRICULARIA - bladder development
Utricularia sandersonii, shoot tip with a bladder primordium.
Bladder development and growth run in principle similarly in all species. See the page on bladder function before continuing here. The initiation is illustrated by U. sandersonii as an example. In this species, the bladders develop as modified leaves from the runners while they originate from modified leaf segments in the free-floating aquatic species. The bladder primordium is seen as a small hump in the runners elongation zone (left Fig.).
Initial invagination of bladder.
By asymmetrical growth a small groove soon appears near the top of the hump (right Fig.). That is the beginning of an invagination of the upper (adaxial) side of the leaf, which finally covers the inside of the bladder while the lower leaf side (abaxial) constitutes the outside of the bladder. The bladder wall consists of only two cell layers interpreted as the epidermis of the ad- and abaxial side of the leaf.
Utricularia sandersonii, bladder invagination.
Same seen from the side.
In the Fig. at left, a bladder stalk has developed and the invagination is in progress. A few immature slime glands occur on the bladder stalk. The invagination rapidly narrows to a narrow crack, and the bladder becomes closed while it is growing. In the longitudinal section shown to the right, a triangular cell is located opposite the cells which will form the threshold. The triangular cell will divide several times and produce the door, which in the beginning will grow directly into the bladder lumen parallel with the threshold. The rounded black bodies are nucleoles within the cell nuclei.
Same stage seen in median longitudinal section.
Utricularia sandersonii, early stage in threshold development
U. sandersonii, early stage in vestibule (lobby) development.
In the stage of development shown to the right cell divisions are still going on in the sides of the bladder and elongation growth occurs in the remaining cells, hence the lumen is increasing. Simultaneously the door expands in parallel with the threshold, and the triangular cell at the tip of the door (later its bottom edge) continues to divide. The strongly vacuolated cells are close to maturity, while the cytoplasm rich cells in the door and threshold will divide more times.
On the Fig. to the left, the first hydathodes are developing on the outside of the bladder wall. Also notice the ‘set of teeth’ along the invagination. It represents the tender beginning to the alluring glands of the vestibule. The threshold is broader and still grows simultaneously with the door. The triangular cell is clearly visible in the Fig. to the right.
U. lateriflora, threshold and door in a median longitudinal section.
U. lateriflora, the door continues to grow into the bladder lumen and the threshold becomes broader.
Utricularia sandersonii, early growth stage of alluring trichomes
Utricularia sandersonii, alluring trichomes still growing.
In these Figs. ‘the set of teeth’ is growing and it finally develops into long-stalked alluring trichomes with a small spherical gland-head. Simultaneously with the elongation of the alluring trichomes, the vestibule (lobby) opens and inside the bladder, the door begins to raise in relation to the threshold. Here the glandular hairs of the epithelium begin developing (see here). At the same time, more hydathodes become present on the outside of the bladder, although in U. sandersonii the number is modest.
Utricularia sandersonii, the vestibule (lobby) opens.
Utricularia sandersonii, fully developed vestibule with panels of alluring trichomes in front of a circular door.
In these Figs, the bladder is fully developed and about one mm long. The alluring trichomes of the vestibule are arranged in panels/curtains and thereby they get further two functions. They obtain the same alluring guiding function as the trap-hairs in free-floating species but probably more importantly, they form a sort of lattice or filter to counteract larger prey in getting so deeply into the vestibule that the door is blocked and unable to function. This may be the case on the right picture below. In U. sandersonii, the door raises only to an angel of 45° in relation to the threshold. In other species, the door stands perpendicularly to the threshold (see more here). At the right picture below, some of the four-armed digestive glands inside the bladder are visible.
Utricularia sandersonii, mature bladder (1 mm).
Utricularia sandersonii, mature bladder seen in side view, about 1 mm long.
Utricularia sandersonii, median longitudinal section of mature bladder. Prey is pinched in the door.–
H. S. Heide-Jørgensen February 2021.