midrib vs stratification

midrib

noun
  • The continuation of the petiole of a pinnately compound leaf around which the leaflets attach. 

  • The strengthened vein down the middle of a flower petal or simple leaf or leaflet. 

stratification

noun
  • the vertical layering of vegetation in a forest 

  • society's categorization of people into socioeconomic strata 

  • process of treating seeds to simulate natural conditions that the seeds must experience before germination can occur; most often the process involves exposure to cold in which case it may also be called cold treatment 

  • the process leading to the formation or deposition of layers, especially of sedimentary rocks 

  • a layering of musical texture 

How often have the words midrib and stratification occurred in a corpus of books? (source: Google Ngram Viewer )