Read it through once

The difficulty is very often to know where to draw the line. Probably no flowering plant is quite independent of the labour and work of its neighbours. As we have tried to show in another chapter, a long preliminary cultivation by bacteria, lichens, and mosses is required before flowering plants can develop on bare rock. That is also necessary in all cases where the soil is mineral or _inorganic_, without any _organic_ dust or fragments of vegetable or animal matter. Bacteria must always begin the work by preparing nitrates and other salts.