THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
The fungi are a distinct kingdom of organisms, comprising about 74,000 named species. Mycologists, scientists who study fungi, believe there may be many more species in existence. Although fungi were at one time included in the plant kingdom, they lack chlorophyll and resemble plants only in their general appearance and lack of mobility. Significant differences between fungi and plants include the following:
Fungi are heterotrophs. Perhaps most obviously, a mushroom is not green because it does not contain chlorophyll. Virtually all plants are photosynthesizers, whereas no fungi carry out photosynthesis. Instead, fungi obtain their food by secreting digestive enzymes onto whatever they are attached to and then absorbing into their bodies the organic molecules that are released by the enzymes.
Fungi have filamentous bodies. A plant is built of groups of functionally different cells called tissues, with different parts typically made of several different tissues. Fungi by contrast are basically filamentous in their growth form (that is, their bodies consist entirely of cells organized into long, slender filaments called hyphae), even though these filaments may be packed together to form a mass, called a mycelium (figure 18.1).
Figure 18.1. Masses of hyphae form mycelia.
The body of a fungus is composed of strings of cells called hyphae that pack together to form a mycelium like the dense, interwoven mat you see here growing through leaves on a forest floor in Maryland. Most of the body of a fungus is occupied by its mycelium.
Fungi have nonmotile sperm. Some plants have motile sperm with flagella. The majority of fungi do not.
Fungi have cell walls made of chitin. The cell walls of fungi contain chitin, the same tough material that a crab shell is made of. The cell walls of plants are made of cellulose, also a strong building material. Chitin, however, is far more resistant to microbial degradation than is cellulose.
Fungi have nuclear mitosis. Mitosis in fungi is different than mitosis in plants and most other eukaryotes in one key respect: The nuclear envelope does not break down and reform; instead, all of mitosis takes place within the nucleus. A spindle apparatus forms there, dragging chromosomes to opposite poles of the nucleus (not the cell, as in all other eukaryotes).
You could build a much longer list, but already the take-home lesson is clear: Fungi are not like plants at all! Their many unique features are strong evidence that fungi are not closely related to any other group of organisms.
The Body of a Fungus
Fungi exist mainly in the form of slender filaments, barely visible with the naked eye, called hyphae (singular, hypha). A hypha is basically a long string of cells. Different hyphae then associate with each other to form a much larger structure, like the shelf fungus you see growing on a tree in figure 18.2.
Figure 18.2. A shelf fungus, Trametes versicolor.
The main body of a fungus is not the mushroom, which is a temporary reproductive structure, but rather the extensive network of fine hyphae that penetrate the soil, wood, or flesh in which the fungus is growing. A mass of hyphae is called a mycelium (plural, mycelia) and may contain many meters of individual hyphae.
Fungal cells are able to exhibit a high degree of communication within such structures, because although most cells of fungal hyphae are separated by cross-walls called septa (singular, septum), these septa rarely form a complete barrier, and as a consequence, cytoplasm is able to flow from one cell to another throughout the hyphae. The photo in figure 18.3 shows a junction between two fungal cells and the septum that partially separates them. From one fungal cell to the next, cytoplasm flows, streaming freely down the hypha through openings in the septa. Keep in mind the differences in scale between the hyphae and mycelia; the hypha in figure 18.3 is about 3.4 pm across whereas the mycelium in figure 18.1 is visible with the naked eye.
Figure 18.3. Septum and pore between cells in a hypha.
This photomicrograph clearly shows cytoplasmic streaming through a pore in the septum between two adjacent cells of a fungal hypha.
Because of such cytoplasmic streaming, proteins synthesized throughout the hyphae can be carried to the hyphal tips. This novel body plan is perhaps the most important innovation of the fungal kingdom. As a result of it, fungi can respond quickly to environmental changes, growing very rapidly when food and water are plentiful and the temperature is optimal. This body organization creates a unique relationship between the fungus and its environment. All parts of the fungal body are metabolically active, secreting digestive enzymes and actively attempting to digest and absorb any organic material with which the fungus comes in contact.
Also due to cytoplasmic streaming, many nuclei may be connected by the shared cytoplasm of a fungal mycelium. None of them (except for reproductive cells) are isolated in any one cell; all of them are linked cytoplasmically with every cell of the mycelium. Indeed, the entire concept of multicel-lularity takes on a new meaning among the fungi, the ultimate communal sharers among the multicellular organisms.
Key Learning Outcome 18.2. Fungi are not at all like plants. The fungal body is basically long strings of cells, often interconnected.