Foliar trichomes

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The second absorptive organ mentioned earlier is the trichomed leaf. Although several epiphytic ferns, Astelia, and perhaps some orchids bear absorptive foliar hairs, the trichome of Bromeliaceae, particularly of subfamily Tillandsioideae, exhibits the most elaborate structure. Scales (another term for these peltate hairs) feature shields made up of empty cells

Foliar Trichomes
Figure 3.21. An absorbing bromeliad trichome in the dry and wet configuration. Arrows depict the route of water and solute movement into the mesophyll in the wet specimen.

aligned in orderly fashion (Figs. 3.6, 3.21). Centermost, just over the point where the dome cell subtends the shield, are four equal-sized, thick-walled, empty cells. Several additional rings of dead cells, each made up of twice as many cells as the one before, may be present. The outermost portion of the shield forms the wings, containing over twice as many, and now more elongated, cells as the previous ring. Ring cells have thin zones in outer walls that allow wing flexure. Thus a trichomed bromeliad leaf is, like the vela-mentous root, bounded by an absorbent, nonliving tissue which captures flowing moisture for slower absorption into living tissue.

Most tillandsioid trichome stalks are composed of three to five transfer cells supported by two foot cells. The dome cell is the largest of the chain and contains a dense protoplast with a prominent nucleus. Dolzmann (1964, 1965) noted that it is equipped with an elaborated plasmalemma plus other membrane systems and numerous mitochondria which he theorized are needed for absorption. Walls of both shield and stalk cells are modified in a pattern consistent with that critical function. All transverse junctions between cells - disc-dome, dome-stalk, intrastalk, and stalk-foot - are uncutinized and penetrated by numerous plasmodesmata. Shields are variously shaped and oriented (Figs. 3.6, 3.7, 3.11). Those on leaf surfaces of shade-adapted species are small, immobile, and scattered (Fig. 3.10); if broad and numerous, they are permanently appressed against the epidermis to reduce reflectance (e.g., Tillandsia bulbosa; Benzing et al. 1978). The foliage of species regularly subjected to clouds, fog, or high humidity tends to be slender and elongated (Fig. 3.11). A rough-textured silvery shoot is created by trichome shields that are elevated and extended as if to dissipate excess surface moisture and avoid the threat of suffocation. Indumenta on the most xerophytic forms maintain a flat immobile layer over stomata.

Although other theories have been offered (Haberlandt 1914; Dolzmann 1964, 1965), water absorption through a bromeliad scale can be described plausibly in terms of osmotic and mechanical forces alone (Fig. 3.21). Early workers, including Haberlandt (1914) and Mez (1904), who observed that plasmolysis occurred first in mesophyll cells surrounding stalk bases when hypertonic solutions were applied to intact leaf surfaces, correctly evaluated the tillandsioid trichome's absorptive power. Rather than beading up, drops of moisture falling on a leaf quickly spread by capillarity to form a thin uniform film between the trichome shields and foliar epidermis. Shield cells readily imbibe fluid; as filling proceeds, the upper walls of the central disc are forced upward while the wing flattens against the leaf surface. By the time emptying is completed, shield cell lumina have pinched shut. Once again, a thick plug is interposed between dome cell and bulk air, and water is prevented from leaving the plant by its route of entry. In effect, the atmospheric bromeliad's trichome serves as both a one-way valve and an energy dissipator, alternately hydrating the plant and insulating it against water loss and intense insolation. Functional analogy between this sytem and the vela-men/exodermis is obvious, although the latter is less dynamic.

The absorptive role of trichomes on leaf sheaths of tank bromelioids has not been investigated adequately. Also, claims that foliar hairs of several polypodiaceous ferns can supplement root function are based solely on capacity to take up eosine (Mliller et al. 1981). Pleurothallid orchid leaves bearing hairs with similar qualities showed very little rehydration capacity. Atmospheric bromeliads behaved quite differently. Leaves of these taxa, if draughted to 20-30% deficits, could still rehydrate in a matter of hours after wetting. They did not respond, however, to water-saturated air, contrary to other experimental findings (Fig. 3.22; DeSanto, Alfani, and DeLuca 1976). Garth (1964) demonstrated that Tillandsia usneoides required contact with water to survive in a Georgia forest. Terrestrial Tillandsia purpurea and T.

Streptophylla

o e. tampensis 60 ■ • t. streptophylla o r. ophiocephala start 100% rh start mist

1 cc

o e. tampensis 60 ■ • t. streptophylla o r. ophiocephala start 100% rh start mist

days

Figure 3.22. Rehydration of partially desiccated leaf sections of two orchids and the atmospheric bromeliad Tillandsia streptophylla during exposure to water-saturated air and liquid moisture. (From Benzing and Pridgeon 1983.)

latifolia flourish in Peruvian coastal deserts where fog rather than rain serves as the moisture source except during the pluvial El Niño years. Water vapor as opposed to rain or fog is probably much more important to epiphytes as a transpiration retardant than as a direct source of tissue moisture.

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