[R] fitting a gompertz model through the origin using nls

[Swantje Löbel]

Dear all!

I tried to fit Gompertz growth models to describe cummulative germination rates
using nls. I used the following code:

germ.model<-nls(percent.germ~a*exp(-b*exp(-k*day)),data=tab,start=list(a=100,b=10,k=0.5))

My problem is that I want that the fitted model goes through the origin, since
germination cannot start before the experiment was started, and y-max should be
100.

Does anyone know how I can achieve this?

Thanks a lot in advance! Swantje