The following points highlight the two types of conidia found on free conidiophores. The types are: 1. Thallospores 2. Conidiospores.
Type # 1. Thallospores:
These are asexual spores formed by the transformation of existing cells of the thallus. These spores are set free by decay or disarticulation of the parent hyphae but not by a process of abstraction.
Thallospores may be terminal or intercalary in position and may be of two types:
(i) Arthrospores:
Arthrospores arise by close septation of the distal part of hypha, the septa being formed in basipetal succession. Proceeding from the apex to the base of the hypha each cell is rounded off and is set free as an arthrospore. Coremiella ulmariae produces arthrospores.
(ii) Chlamydospores:
These are formed from a solitary or neighbouring intercalary cells of a hypha, which round off, enlarge and develop a thickened often pigmented wall and dense contents containing food reserves.
Chlamydospores generally function as resting spores and are formed under unfavourable environmental or nutritional conditions. In Trichoderma viride the mature chlamydospores are more or less spherical or oblong and wider than long.
Type # 2. Conidiospores:
These are asexual spores formed singly or in chains on a conidiophore. The sporogenous part of the conidiophore is usually apical but it may be lateral as well.
These spores or conidiospores or conidia as they are commonly called, may be produced in basipetal or ascopetal succession on either free conidiophores or their aggregations in the form of fructifications.
Conidiosphores produced on free condiophores may be of four types:
These are:
(i) Blastospores,
(ii) Porospore,
(iii) Aleuriospores or Gangliospores and
(iv) Phialospores.
(i) Blastospores:
A blastospore or bud spore is formed by budding of somatic cells of a hypha or conidiophore or by budding the cells of other types of spores.
Each bud first appears as a small globular outgrowth from a parent cell and then enlarges while still attached with the parent finally separating by abstraction to form an independent spore.
Blastospores may be formed singly or in ascopetal succession and rarely in basipetal succession. Genera belonging to family Torulaceae like Torula. Bispora, Septomema etc. form typical blastospores.
(ii) Porospores:
Porospores are modified blstospores and are produced as a bud extruded through a distinct pore in the wall of conidiophore.
At the tip of the conidiophore, a pore is enzymatically dissolved in the outer layer of the wall while the inner wall blows out through this pore like a bud and develops into the conidium. Examples of porospores are found in Alternaria, Helminthosporium etc.
(iii) Aleuriospores or Gangliospores:
These spores are formed by the apex of the conidiophore becoming inflated and delimited by a septum at an early stage. In oidium aleuriospores or gangliospores are formed in a basipetal chain by successive division of a meristematic cell at the base of the chain.
The proximal daughter cell remains meristematic while the distal daughter cell becomes an aleuriospore or gangliospore.
(iv) Phialospores:
These are conidia which are abstracted in basipetal sequence from phialides. A phialide is a unicellular structure, oval subcylindrical or flask-shaped with a distinct basal swelling and narrow distal neck and is borne terminally on a simple or branched conidiophores.
In Aspergillus phialospores are dry and persist in chains while in Stachybotrys phialospores remain as slimy solitary spores. Talbot (1972) regards phialospores more as sexual spores (spermatia) rather than as asexual spores.
