Apical extension is a major characteristic of fungi with tubular hyphae. As exploratory organisms, being able to extend and dip their mycelia into the space around them is a crucial part of survival. They produce biomass, such as lipids and proteins, which are delivered to the tips of their filaments via vesicle trafficking, allowing them to add on their ends to keep growing outward. Once a fungus finds something of value, extension stops and branching begins to milk whatever food source they found. Other outwardly extending systems, such as blood vessels in humans, may seem similar, but these traits evolved convergently, such being that there are only so many ways to branch out radially with autotropism, or avoiding bumping into yourself. On a side note, sometimes positive autotropism is needed, specifically for hyphal fusions, but this is not the case most of the time.

Extension and growth are two different concepts in mycology. Extension means the hyphal tips are being added to, extending the fungus further into the substrate. Growth, on the other hand, has to do with the production of biomass that happens all along the hypha and not just the tip. Sections 4.4 and 4.9 get into the mathematics of fungal growth kinetics, so I will just briefly cover some of the main points. One thing to consider is that if there is an increase in biomass but no extension, AKA the borders of the fungi have not increased, then branching is occurring. Also, the total mycelial length and number of apices increase exponentially at the same rate, which is also the specific growth rate of the organism under the same favorable, excess conditions. Branching is triggered when the average volume of cytoplasm exceeds a certain value. 

The prior mentioned concepts, unfortunately, have limitations. These things were studied in young fungi with unrestricted access to resources. In real life, there are three possible growth phases: lag, exponential, and deceleration, with the latter of these being a slowed, restricted growth. Less favorable conditions, unfortunately, lead to this restricted growth, as does programmed senescence. Additionally, hyphae are not one size fits all. As hyphae age and extend and live their lives, they differentiate into up to four different types that have different roles. The oldest type is the ageing zone, which is a sporulating portion, then the fruiting zone, productive zone, and peripheral zone. Expectedly, the peripheral zone grows exponentially and absorbs the most nutrients, while each zone working backwards absorbs less and less. 

Traditional cell separation does not occur in filamentous fungi. From what I understand, new growth occurs and a septa, or cross-wall, is produced to separate it into compartments. A size detecting mechanism looks for the ratio of cytoplasm to number of nuclei and once it hits a certain point, more divisions take place. More than one nucleus can be present in each division and one spindle does not equal one septum, as is the rule with animals and plants. These nuclei can travel throughout the mycelium and some species even see a migratory cycle. First, the content of the cells move to the apex while the apex also is extending. The distance between the nucleus and the tip remains somewhat constant since both are moving. The second phase occurs when the nucleus stops moving and more space grows. Phase three is synchronous mitosis, then in phase four, one of the nuclei starts moving towards the apex faster than the apex is moving to get closer while the other moves the other direction. The fungi need a strong microtubule system to accomplish this feat. Personally, I am wondering if an antifungal drug could be developed which could disturb the movement of the nuclei. No idea if this exists, but I couldn’t find much with a quick Google search.

Septa can divide hyphae completely, with penetration, or with perforation. The first is a solid wall, the second has cytoplasmic strands running through it, and the last has a pore. Often septa with pores have a parenthesome, which acts like a little cap to protect it. Between the way the cells are divided to some having literal holes in them, some scientists argue if these compartments are able to be called individual cells. The book (and I) consider these to be cells since they function as them, but they do get close to that semantic dividing line. 

Lastly, spores can be produced sexually, asexually, or both depending on the species. Rather than produce seeds or, for lack of better words, a baby, fungi instead use cells to reproduce and disperse. A benefit of this is that many species’ spores can go dormant to survive difficult situations. Exogenous spores will emerge from their dormant state when the environment becomes tolerable again. Endogenous spores, on the other hand, require a certain amount of time or to experience a shock to wake up. These strategies are very clever ways to maximize their chances of survival. Exogenous species don’t take chances with risky situations, while endogenous species may wait until, say, after a forest fire, to reëmerge because they will have easy pickings of now dead organisms to decompose. Before leaving their dormant phase, they swell, typically from absorbing water, then form germ tubes. The hope of this chapter was to explain the mechanisms of how fungi are able to inspect their surroundings in order to find sustenance and nutrients.

Happy reading,
-Beppa