Question

The upper and lower layers of the leaf are formed by epidermal cells, with a waxy cuticle covering the outer surface. The cuticle protects the epidermis and reduces water loss. Below the upper epidermis are the palisade mesophyll cells, the primary site of photosynthesis in the leaf. Below the palisade layer is the spongy mesophyll layer, consisting of loosely packed photosynthetic cells. The loose arrange­ment of the spongy mesophyll allows for high uptake of carbon dioxide and release of oxygen across the exposed surface of the photosynthetic cells. The lower epidermis con­tains microscopic openings or stomata that allow gas exchange with the atmosphere.

Although lichen is often mistaken for moss, it is not a plant! In fact, it is not even closely related to plants. Despite this, it is capable of photosynthesis.

Lichens are formed by a mutualistic association between a fungus and a photosynthetic organism. The photosynthetic component may be a chlorophyte (green alga) or a cyanobacterium. The upper and lower cortex of the lichen is made of tightly compressed fungal hyphae. (Hyphae is the main component of fungi. It is the ‘fuzz’ that makes bread mold look fuzzy. Mushrooms are fungal reproductive structures formed by very densely packed hyphae.) Loosely packed hyphae form the middle layer or medulla. The photosynthetic cells are held among hyphae in the upper medulla.

a) For each of the following, explain how their form (i.e., the structure or organization) is similar, and explain how this helps with function.

1. The spongy mesophyll of the plant leaf and the medulla of the lichen.
2. The location of the photosynthetic cells

b) What evolutionary process accounts for the similarity between these two groups? (one word answer)

c) Do these similarities provide information about the evolutionary relationships of lichens and plants? Why or why not?

Answer 1

a) Most kinds of lichens have a similar organization of their cells. There is an upper cortex of fungal cells, beneath which the algal cells are located. Below the algal cells is an area of loosely organized fungal cells called the medulla. Beneath the medulla is a lower
cortex of fungal cells. Cortices are often covered with cuticle or some other protective material.
The above descriptions reveal striking similarities between a leaf and lichen. The epidermal layers of the leaf and the cortices of the lichen provide protection. The photosynthesizing cells are just below the upper
surface in both leaf and lichen. Furthermore, the
spongy mesophyll of the leaf and the medulla of the lichen consist of loosely arranged cells,which allow for a ready exchange of carbon dioxide and oxygen between the photosynthesizing cells and the atmosphere.

b) Convergent evolution