Every now and then, while doing research, one runs into little gems of history. In the case of Datura Stramonium, I found a thesis by Isao Nishimura at the University of Wisconsin dated 1922. The thesis contains meticulous descriptions of experiments conducted to unravel the mysteries of Datura Stramonium.
The researcher prepared some microscopic drawings of the seeds, pointing out that earlier researchers had stated that the alkaloids were present in the outer layer, and that the endosperm and the embryo are free from alkaloids, although one researcher reported trace amounts of alkaloids.
To detect the presence of alkaloids a solution of iodine in potassium iodide was used to stain the alkaloids, even though it also reacts with starches.
Visual inspection showed that the endosperm and the embryo were wholly free of staining, showing the absence of alkaloids in these parts of the seeds.
The thesis also discusses the oils extracted from mature and immature seeds, observing that the former were yellowish in colour while the latter one was dark green.