[2] While studying he was offered a job at the Rijksherbarium [nl], the herbarium of Leiden University, by its director Cornelis Gijsbert Gerrit Jan van Steenis.

[3] For his final year of studying Baas wished to stay at the Royal Botanic Gardens, Kew.

[2] Between 1968 and 1969 Baas studied at the Jodrell Laboratory of the Royal Botanic Gardens under Professor Charles Russell Metcalfe.

[4][5] On his return from the United Kingdom Baas approached Van Steenis and asked to be employed as a wood anatomy expert.

[1][6] In 1987 he became professor (Bijzonder hoogleraar [nl], paid from non-university funds) of plant systematics at Leiden University.

Although content as a researcher and not very interested in directing and managing, Baas took up the position of director out of a sense of duty.

[3] Two years after starting as director, the Rijksherbarium was faced with a plan of the dean of the University Faculty of Mathematics and Natural Sciences to slash the budget by half, which would have forced Baas to fire all scientific staff.

[3] The university board and the Royal Netherlands Academy of Arts and Sciences aided Baas in his wish to see the collection preserved and a special fund was established.

[6] During his term as director, Baas managed to improve digitalization efforts and nature conservancy projects at the institute.

[3] As of 2013 he was still active as professor emeritus and honorary staff member at the Naturalis Biodiversity Center, the successor institute to the National Herbarium of the Netherlands.

[4] Baas's principal research was in the evolution of anatomical diversity in wood and in the significance of tree biology as it relates to global environmental change.