The word thalli comes from the 19th century Greek words thallos meaning green shoot and thallein meaning to bloom or to sprout. 
In contemporary botany, thalli or thallus refers to the undifferentiated cells of ‘lower class’ organisms such as algae (including seaweeds), fungi, lichens, and liverworts.  
The (analogous) tissue of undifferentiated cells in plants can be referred to as the meristem. The meristem is composed of cells which continually divide and allow the plant to grow. Plants have two, or more, meristems: one at the downward-reaching root and the other the upward-reaching tip. 
So why are we talking about thalli and meristems? As gastronauts, the thallus and the meristem are important because that's where the freshest, tastiest, most tender growth is. As mentioned already fungi, or mushrooms, and algae, including seaweeds, are examples of thalli. All of these food sources are of special interest to us since we are driven to find diversity.
We are continually searching for plants at the (perpetual) peak of their growing potential: when they are most supple, most delicious, most nutritious and easiest to digest.
thalli (thal-ahy) /ˈθælaɪ/ noun. singular: thallus (thal-uh s) (ˈθæləs) Botany: A plant body that is not differentiated into stem and leaves and lacks true roots and a vascular system. Thalli are typical of algae, fungi, lichens, and some liverworts. Derived Forms: thalloid, adjective Origin: Greek; from thallos green shoot; from thallein to bloom, to sprout
1. Simpson, J. A., E. S. C. Weiner, and Michael Proffitt. Oxford English Dictionary. Oxford: Clarendon, 1993. Print. 2. Haupt, Arthur W. Plant Morphology. New York: McGraw-Hill, 1953. Print. 3. Encyclopædia Britannica." Wikipedia. Wikimedia Foundation, n.d. Web. 18 Nov. 2016. 4. Rouse, Susan T. "Meristems." Biology Reference. N.p., n.d. Web. 18 Nov. 2016.