Further Development 1.7: Transitions of the Wall, the Foot, and the Tube

The Making of a Body and a Field: A Framework for Understanding Animal Development

Several additional innovations made terrestrial life possible for plants. One of the most significant was the synthesis of tough fibers of the polysaccharide cellulose, used to build the terrestrial plant cell wall. Further adaptations of the cell wall—such as in the alignment of the cellulose fibers and the addition of other carbohydrates and proteins—yielded more rigid walls with greater protection against water loss and damage from ultraviolet radiation, as well as greater support for upright growth (see textbook Figure 1.25, step 17; Popper et al. 2011; Mikkelsen et al. 2014). However, upward growth would not be possible without a secure footing on the ground. Thus, another key step in the developmental evolution toward land plants was to establish differential specialization along the top (apical) to bottom (basal) axis of the plant, allowing basal cells to evolve into structures capable of “rooting” to the ground. Further evolution of these anchoring rhizoids also provided entrance points for nutrients (see textbook Figure 1.25, steps 19–20; Jones and Dolan 2012).

Adaptations to the mechanisms facilitating the transport of nutrients marked yet another advancement that fueled increases in plant size. The foundational innovations of the plasmodesmata (open channels between cells for the exchange of large substances) and the stomata (a functional gate for water and gas exchange) set the stage for the evolution of more complex transport mechanisms (see textbook Figure 1.25, step 24, and Figure 1B,C). However, it was the bryophytes (mosses and liverworts; see Figure 1D)—the first significant group of plants to colonize land—that developed rudimentary vessel-like tubes for nutrient movement (see Figure 1E). These tubes then evolved into the more complex xylem and phloem of today’s vascular sporophytes, from fern to Fraser fir (see Figure 1F). This vascular tissue enabled the long-range movement of water and sugars throughout the plant, which was required if indeterminate growth was to be possible (textbook Figure 1.25, steps 25–30).

**Figure 1** Transitional states over the course of plant evolution. (A) Illustration of *Chara braunii*, an extant species of charophytic alga. The reproductive organs—the oogonium (oocyte) and antheridium (sperm)—are illustrated and shown to the left. The rhizoids are illustrated to the right. (B) Transmission electron micrograph of plasmodesmata (arrow) in the charophytic alga *Chara zeylanica*. (C) Pseudocolored scanning electron micrograph of stomata (arrows) on the leaf of a coriander plant. ?(D) A bryophyte, the rooftop moss *Dicranoweisia cirrata*, with capsules on the tips of the setae. (E) Hydroid and leptoid cells. (F) Image of *Pteridium aquilinum*, a fern. To its right is a single transverse section of the middle stem region and two views of a high-resolution computed tomography volume rendering of xylem in this section of the stem. (G) Developmental stages of the cork oak acorn (S4–S8). Notice how the cupule retreats as the pericarp expands during seed maturation. (A, after T. Nishiyama et al. 2018. *Cell* 174: 448–464p>

Literature Cited

Jones, V. A. and L. Dolan. 2012. The evolution of root hairs and rhizoids. Ann. Bot. 110: 205–212.

Mikkelsen, M. D., J. Harholt, P. Ulvskov, I. E. Johansen, J. U. Fangel, M. S. Doblin, A. Bacic and W. G. Willats. 2014. Evidence for land plant cell wall biosynthetic mechanisms in charophyte green algae. Ann. Bot. 114: 1217–1236.

Popper, Z. A., G. Michel, C. Hervé, D. S. Domozych, W. G. T. Willats, M. G. Tuohy, B.Kloareg and D. B. Stengel. 2011. Evolution and diversity of plant cell walls: From algae to flowering plants. Ann. Rev. Plant Biol. 62: 567–590.

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