However, we can define the fungi broadly in terms of lifestyle and accommodate all these organisms. Hyphae are surrounded by a wall and extend at their tips, while drawing the protoplasm forwards as they grow. The hyphae branch repeatedly to form the 'body' of a fungus, termed the mycelium . The left-hand image at the top of this page shows several examples of this. The only major departure from the mycelial growth form is found in the unicellular yeasts , but even yeasts can produce hyphae in appropriate conditions.
All fungi depend on pre-formed organic nutrients for energy and for synthesis of cellular materials.
Figure A. Tip growth of fungal hyphae. These three images from a videotape sequence were taken over a period of 20 minutes after a glass coverslip was placed on the margin of a colony of Neurospora crassa . The left-hand image is typical of a hypha, with extension growth at the hyphal apex, and branches arising from points behind the main apex. It enables them to grow continuously into fresh zones of nutrients and also to penetrate hard surfaces such as plant cell walls, insect cuticle, etc. This is why fungi are so important as plant pathogens and as decomposer organisms.