@staffan No. I am not referring to the Mula Nakshatra or Nakshatras in general. At present, I am taking the New Rasi Sutras class by Mr. Wilhelm and in Class 16 of the course he talks about a specific principle of classifying the Rasis based on the information present in Jataka Parijata. Accordingly, the Rasis may be classified as Dhatu Rasis, Mula Rasis, and Jiva Rasis. Dhatu refers to minerals. Therefore, all inanimate objects or non-living things. Mula refers to things with roots. Hence, it primarily denotes the plant kingdom and derivatives from it. Jiva Rasis are living creatures with all the five senses. The classificatory scheme progresses in the following order across the twelve zodiac signs: Dhatu --> Mula --> Jiva. So, Aries is Dhatu, Taurus is Mula, and Gemini is Jiva. In this classificatory scheme you would notice that all the movable signs belong are Dhatus, the Fixed signs are Mula, and the dual signs are Jiva.
Mr. Wilhelm does mention that this classficatory scheme is applicable to the Nakshatras as well, like you mention. With regard to the Nakshatras, he claims that Dhatu-Mula-Jiva affects the very behaviour of a person.
However, when it comes to the Rasis, this classificatory scheme helps to determine how the surrounding context of a person or the consciousness surrounding a native affects his or her ability to do something. In other words, is a person overwhelmed by their context or is he or she able to overcome the same or at least remain not distracted by it and successfully pursue his or her activity.
At least this is what I understood.
Plants or Mula as mentioned by Mr. Wilhelm cannot survive in extreme environments. But, certain plants do grow I think in deserts and even at mountain peaks, if I am not mistaken. Although they are much less sensitive to the external conditions in comparison to the Jiva Rasis, they have some degree of consciousness. Furthermore, since Mula are entities with roots, I was thinking if they could also indicate this tendency 'to go to the bottom of things' or 'roots of things'.
Best,
Krishnamurari