Fig 1.
False indusia on A. peruvianum.
(A) A leaflet with several pinnules. (B) The false indusia are being developed on the underside of the pinnule. Several developmental stages of false indusia (note the colors) are shown. The lamina of older pinnules with open false indusia shows signs of degradation (lowermost image). (C) Mature false indusia open completely by desiccation and the exposed sporangia release their spores.
Fig 2.
Leptosporangia on the underside of a false indusium of A. peruvianum.
(A) Scanning electron microscopy (SEM) image of a ruptured empty sporangium. The inset shows a trilete spore. (B) Light microscopy (LM) image of a ruptured empty sporangium. The inset shows annular cells with gas bubbles that occurred due to cavitation inside the cells. (C) Underside of a detached false indusium with sporangia that partly have already shed their spores. The connection line to the pinnule margin is indicated.
Fig 3.
Non-mature and mature false indusia and supplying pinnule veinlets.
(A) When non-mature false indusia desiccate (by cutting off the respective pinnule margins), they do not open completely but remain in a semi-open position. No spores become shed (see also S1 Video). (B) View on the adaxial, marginal surface of a pinnule. The connection line between a false indusium, which is situated underneath the lamina, and the pinnule is indicated. The veinlets supplying the false indusium are clearly visible. (C) Details of some of the open and of some of the still closed false indusia after the desiccation experiment described in paragraph 3b). (D) Pinnules with open false indusia show signs of desiccation and degradation in the vicinity of the false indusia (indicated by the arrow).
Fig 4.
Kinematics of false indusia situated on cut-off pinnule margins.
(A) Motion of a false indusium recorded from lateral, frontal and top view in our microscopy lab (~23° Celsius, ~50%rh). The false indusium performs a curvature inversion, rotates around its connection line to the pinnule, and its lateral regions flap towards the middle part. (B) Motion of another false indusium in greater detail. (C-E) When the false indusium is in its fully open state (as seen here from different angles), the sporangia (here empty already) point into different directions, an arrangement for which we hypothesize that it promotes spore scattering. (F) The desiccation-driven opening motion is reversible because open false indusia completely re-close under water.
Fig 5.
Loss of mass during the movement of a detached false indusium.
The movement of the false indusium could not be quantified and is, therefore, qualitatively depicted by presenting single frames in 40 min time steps (the respective S3 Video is recorded with a speed of 1 frame per 5 minutes). The motion is continuous and accompanied by an asymptotic mass loss of up to ~42%.
Fig 6.
Cellular changes taking place during desiccation of a false indusium.
(A-B) LM transverse sections of false indusia in the closed (A) and open (B) state. It is functionally built as a bilayer, with the adaxial layer comprising a single row of large tubular cells and the abaxial functional layer consisting of three rows of tangentially elongated, smaller cells. Especially the adaxial, tubular cells undergo a marked deformation during dehydration. The scale in (A) and (B) is identical. (C-D) The collapse of the adaxial cell walls of the tubular cells (i.e, their curvature inversion towards the cell lumen) is well visible after desiccation (S4 Video). The scale in (C) and (D) is identical.
Fig 7.
Sporangium motion sequences and spore dissemination.
The sporangia have been gripped at their stalks with tweezers and adjusted vertically. Sporangium 2 in (D) is still situated on a false indusium. ac = apical cup; bc = basal cup; s1 = spores from apical cup; s2 = spores from basal cup. (A) The slow passive nastic movement of the annulus includes rupture of the sporangium (i,ii) and formation of the apical cup and basal cup (iii). The annulus relaxes after this tensioning process (including a first ultrafast relaxation step), but does not immediately reset to its initial position, stopping approximately halfway (iv). Afterwards, the annulus further resets; the extent of this step varies and can lead to a nearly initial annulus curvature, or may stop much earlier. (B) First, ultrafast annulus relaxation step which leads to rapid spore ejection from the apical cup. This step lasts only 40 μs until spores emerge in the respective frame. Afterwards the sporangium bounces and spores from the basal cup also become shed (t = 50–70 μs). Frames were taken from S7 Video, cropped and digitally resized without interpolation. (C) The dissemination pattern during spore ejection from a sporangium is shown. At the beginning, the tensioned annulus is turned in such a way that the apical cup points left backwards. After relaxation, spores of the apical cup are forcibly ejected almost straight downwards, whereas those from the basal cup are shed in an irregular pattern. Frames from S11 Video were cropped and digitally resized without interpolation. (D) Tracked spores from S9 and S10 Videos (images rotated 90° anti-clockwise). Still frames of sporangium 1 and 2 at t = 0, which depict the last frames before the respective annuli start to move, and at t = 0.5 ms respective t = 0.3 ms. The six tracked spores are visible and marked. Spore 5 from the basal cup of sporangium 2 is already visible at t = 0.
Fig 8.
Scattering pattern of spores, sporangia and sporangium fragments.
(A) Dissemination pattern of a single false indusium. The arrows indicate sporangial fragments or spore clumps. (B) Left images: Spore clumps observed during the experiments. Right images: Sporangial fragments with spores. (C) During recordings for kinematic analyses we often witnessed the detachment of whole sporangia from the respective false indusium (S13 Video).