Systematically larger char yields were observed from the pyrolysis of chemically isolated lignins, compared to expected yields from the pyrolysis of lignins embedded in plant material. Naturally occurring lignins are known to be intermeshed with other plant constituents within the composite matrices of lignocellulosic biomass. An attempt was made to simulate their behavior by pyrolyzing pellets prepared from mixtures of lignin and cellulose powders. However, the results gave char yield trends that did not conform to trends observed when pyrolyzing plant derived biomass. These findings are interpreted in terms of entirely different reaction pathways operating when lignins are pyrolyzed within naturally occurring biomass, compared to pure lignins or composite particles made from mixtures of fine powders. It appears that char yield trends from the pyrolysis of lignocellulosic biomass are closely linked to the detailed morphology, as well as the chemical makeup, of the highly oxygenated plant derived material within which the lignin components of plants are embedded. The observed sensitivity of reaction pathways to plant specific structural (morphological) features poses added challenges in formulating realistic ab initio mathematical models for predicting the pyrolysis chemistry of lignocellulosic biomass.