Read it through once

Each grass has its own special favourite amount of moisture. It likes to have its water at just one particular depth below the surface. Unfortunately there are not nearly enough sympathetic and careful observations of the preferences of each individual grass. A Danish author has worked out the facts in certain localities (Geest). Suppose first that the water-level of the wells, etc., is 6-1/2 to 9-3/4 feet below the surface. This suits the Meadow Poa grass (_Poa pratensis_) exactly. It will grow luxuriantly and flourish. Now suppose the weather is very wet, so that the water rises in the wells till they are three to four feet deep. The Roughish Poa (_P. trivialis_) prefers this moister soil, and it will grow so vigorously that it will kill out the other kind. If it is a season of very heavy floods, or if the drains become choked so that the water rises to within fourteen to twenty-five inches of the surface, then the tufted Aira (_Deschampsia caespitosa_) will kill out the other kinds and flourish abundantly. But if the water rises higher than this the marsh series comes in (see Chap. XVI.).