As presented in Chapter 1,
the generative power of eco-grammar
systems was studied in several papers.
In the original model of simple eco-grammar systems
all the agents have to work in each
derivation step, otherwise the derivation is blocked.
In this section a different derivation mode is examined: in each
derivation step exactly 
agents have to work.
First we summarise the results about the relation between
language classes generated by simple eco-grammar systems with 
agents
working in derivation mode 
. These results appeared in [Csuhaj-Varjú and
KelemenováCsuhaj-Varjú and Kelemenová1998], and
were summarised and completed in [CsimaCsima1997], where
non-extended simple eco-grammar systems were studied.
The notion of extended simple eco-grammar systems
was introduced in [Csuhaj-Varjú, Kelemen, Kelemenová,
and PaunCsuhaj-Varjú et al.1997] and was examined first in
[Dassow and MihalacheDassow and
Mihalache1995]
under the name of 0-terminal EG system.
In the second part of the chapter we study the derivation mode
in extended
systems.
The results, which appeared in [CsimaCsima1998a], show
that unlike the non-extended case, where
the size parameters of the teams and the agent population
have influence on the power of the system, in the extended case
a collapsing hierarchy is obtained.