Estimated Time: 60–80 minutes
Course: Myxomycetes 301 — Microscopic Identification
Lesson Type: Reading + Structural Microscopy Analysis (self-paced)

1. Capillitium Structure & Internal Features

The capillitium is one of the most diagnostically important internal structures in myxomycetes. It consists of a network of thread-like elements within the sporocarp that assist in spore dispersal. Unlike spores, which are relatively uniform in function, capillitium structures vary widely between taxa in branching pattern, thickness, ornamentation, and presence of lime or spiral bands. These differences are often critical for genus- and species-level identification.

Key Takeaway: Capillitium structure is one of the most reliable internal characters for separating myxomycete genera.

2. What is the Capillitium?

The capillitium is a sterile internal network that facilitates spore release through mechanical movement and structural tension.

It is found within the sporocarp alongside spores and may fill much of the internal volume depending on the species.

• Composed of thread-like filaments
• May be free, attached, or connected to a columella
• Often elastic or spring-like in function
• Can contain lime nodes or crystalline deposits
• Highly variable across taxonomic groups

Capillitium architecture is often stable within genera, making it a key taxonomic character.

Branching capillitium network supporting spore dispersal.

3. Major Capillitium Types

Capillitium structures vary widely across myxomycete groups and are often used to distinguish genera.

These structural differences are often visible only under compound microscopy at moderate to high magnification.

4. Elaters and Spiral Ornamentation

Elaters are specialized capillitium elements found primarily in the order Trichiales.

Elaters are spiral-thickened threads that assist in spore dispersal by responding to humidity changes.

• Often coiled or banded
• Respond to humidity changes (hygroscopic movement)
• Help eject or separate spores
• Highly diagnostic in genera such as Trichia and Hemitrichia

The spiral bands may be smooth, rough, or ornamented depending on species.

Various ornamentations of capillitium/elaters; spiral bands, spikes and warts

5. Columella and Internal Support Structures

Some myxomycetes contain a columella, a central sterile structure that anchors or supports the capillitium.

• Can be cylindrical or pointed
• May extend partially or fully through the sporocarp
• Provides structural support for spore mass
• Common in genera such as Stemonitis and Comatricha

The presence, absence, and morphology of the columella are often key diagnostic traits.

Central columella supporting the capillitium network in Stemonitopsis hyperopta.

6. Lime Deposits & Calcium Structures

Many Physarales contain calcium carbonate deposits within the capillitium or surrounding structures.

• Appear as nodules or crystalline coatings
• May obscure internal structures if excessive
• Useful in distinguishing Physarum and Didymium species
• Often visible as bright refractive points under microscopy

Lime content can vary depending on substrate conditions and developmental stage.

Badhamia utricularius — you can see the Badhamiod capillitum in the open sporocarp, bottom left

7. Functional Role of Capillitium

Capillitium structures are not passive—they actively assist in spore dispersal through mechanical movement and environmental responsiveness.

• Helps loosen and release spores
• Responds to humidity changes
• Maintains internal spacing within sporocarp
• Prevents spore clumping
• Facilitates wind dispersal

These functions make capillitium an important evolutionary adaptation in myxomycetes.

8. Comparative Capillitium Examples

• Trichia — spiral elaters
• Hemitrichia — banded elaters with dense ornamentation
• Stemonitis — highly structured network with columella
• Physarum — lime-encrusted capillitium
• Arcyria — delicate, free capillitium threads

Recognizing these patterns helps rapidly narrow identification at the genus level.

9. Common Mistakes in Capillitium Analysis

• Confusing debris with capillitium threads
• Using insufficient magnification
• Examining immature sporocarps
• Overlooking lime structures
• Destroying structure during slide preparation
• Failing to observe multiple specimens

Capillitium interpretation improves significantly with repeated comparative study.

10. Lesson Summary

The capillitium is a defining internal structure in myxomycete taxonomy, providing essential diagnostic information for distinguishing genera and species. Its diversity in form—including elaters, networks, lime deposits, and columella-supported systems—reflects both evolutionary adaptation and taxonomic significance. Mastering capillitium observation is a major step toward advanced microscopic identification skills.

Next Lesson: Case Study — Working Through a Genus Key (Didymium)