Accessory proteins that regulate the nucleation of actin filaments promote binding of the Arp2/3 complex to actin filaments so that most new filaments form as branches from existing ones. These proteins could stimulate Arp2/3 binding to the sides of existing filaments or to the plus end of a growing filament in a way that does not interfere with growth. Both possibilities would yield the final characteristic branched network of filaments. To distinguish between these alternatives, you mix the regulatory proteins with the Arp2/3 complex and actin subunits in the presence of actin filaments that are capped at their plus ends. After a short incubation, you examine the resulting structures by electron microscopy. 

How will this experiment distinguish between these alternatives? 

What structures would you expect to see according to each model for nucleation by the Arp2/3 complex?