Growth, Independence & (Mycelium R&D) Opportunity
I started my journey into mycelium technology from the vantage point of being a visual artist. The critical organizing principle I operated on as an artist was the belief in the power of practicing with intent along the gradient from naive to expert; not waiting until expertise resolved itself before taking my practice with a given subject or skill seriously (a principle of the ‘post-medium’, but we won’t discuss that here). Along this gradient of learning, at a certain point of critical knowledge mass (which occurs fairly early in the learning process), there is enough inertia to begin to behave and create novelly (to actually start making things with novelty and utility). This inflection point is powerful, and very often is more productive and impactful than achieving expertise itself. This critical inflection point along my journey of becoming a mycelium engineer coincided with developing an understanding of the criticality of the independence of specific growth rate and growth velocity.
When two dimensions are independent, it means that variation in one does not predict or constrain variation in the other. In practical terms, changing one variable does not force a change in the other; they can be tuned separately, and their combined effects can create a broader design space.
David Moore reconciles these critical concepts brilliantly in his work 21st Century Guidebook to Fungi. Fungi grow in filamentous form, meaning they expand by extending hyphal tips and by producing biomass (new hyphal material). It is crucial to distinguish growth from extension; specific growth rate refers to how fast the fungal biomass increases, whereas growth velocity refers to how fast the colony expands outward (the linear extension rate of the colony radius). Specific growth rate is about making more mycelial mass, and growth velocity is about how fast the mycelial colony spreads over a surface or through a volume. These two are related but not the same; a mycelial colony can rapidly produce biomass without spreading quickly, or vice versa, depending on how it allocates growth into branching vs. tip extension. This decoupling is a unique feature of filamentous growth and is largely governed by the concept of the hyphal growth unit.
The hyphal growth unit is the average length of hypha (or volume of cytoplasm) per active hyphal tip in the mycelium. It is the total hyphal length divided by the number of hyphal tips. The hyphal growth unit represents the amount of resources supporting each growing tip. A larger hyphal growth unit means each tip has more backing biomass (or cytoplasm) and can potentially extend faster, whereas a smaller value means tips are more numerous relative to biomass. This parameter is not fixed; it can change during colony development. After growth initiates, the hyphal growth unit often increases initially (as the first hypha extends before branching much), then as branching kicks in it may oscillate and eventually stabilize to roughly a constant value once the colony establishes a steady state of growth and branching.
Specific growth rate and growth velocity are interrelated, but they can vary independently if the hyphal growth unit is not constant or if other factors modify colonial expansion independent of the underlying specific growth rate. In an ideal scenario where the hyphal growth unit remains fixed, growth rate and velocity are proportional; speeding up biomass production directly speeds up colony expansion. However, fungi can modulate the hyphal growth unit based on variability in branching frequency.
For example, if the colony encounters a situation where tip extension is hindered, it can continue to produce biomass and will channel that extra biomass into making new branches (increasing tip number) rather than extending length per tip. In this case, colony expansion (velocity) slows down while biomass accumulation continues. Conversely, if conditions favor extension, the fungus may extend with fewer branches, increasing the colony margin even if the total biomass growth rate hasn’t changed. This demonstrates an inherent independence: a colony can expand at a certain linear rate (i.e. distance from colony origin to margin) while its total biomass increases at a different rate.
On its face this is a fundamental principle of fungal growth that is easy to look past, but to a mycelium engineer concerned with extracting value and operable range from fungal colonial behavior, independence between these terms drives to the very core of the opportunity in physical plasticity:
Independence = Tunability, given the breadth of response range accessible between specific growth rate (skeletal volume) and growth velocity (envelope volume).
Deeply appreciating the independence between these two terms is critical to appreciating the opportunity space in fungal physical plasticity, and in my experience, truly was the gateway for enabling a functional inflection from curiosity to practice and accessing a deeper understanding of the principles of physical plasticity as a mycelium design tool chest.