Aspergillus wentii - vesicle with prophialides and phialides
In Aspergillus the conidiogenous cells (phialides) sometimes arise directly from the vesicle (i.e. uniseriate) as in Aspergillus fumigatus (click here) or a short cell (prophialide) arises directly from the vesicle and a cluster of phialides develop from this cell (i.e. biseriate). To identify Aspergillus to species, it is often necessary to establish which of these alternatives is true and it is not always easy. The surest way is to smash the head in water under a coverslip and you can examine the fragments which will include broken off conidiogenous cells.
However, in the photomicrograph above you can see clearly (????) that this species is biseriate. Each prophialide can give rise to a cluster of phialides. You can appreciate, therefore, that the biseriate design gives a much larger diameter head with many more phialides and thus many more conidia can be produced this way.