A stage further is attained in those fungi which enter the stomata and live in the intercellular spaces--_e.g._ many Uredineae and _Phytophthora_--and many such intercellular endophytes increase their attack on the cells by piercing their walls with minute (_Cystopus_) or large and branched (_Peronospora_) haustoria, or even eventually pierce the cells and traverse them bodily (_Pythium_). In all these cases it is clear that conflicts must occur between poison and antidote, acid and alkali, attractive and repellent substances, enzyme and enzyme, etc., as was hinted at above; and the same must take place when the parasite is endophytic and intracellular from the first, as in Chytridiaceae, etc., the zoospores of which pierce the outer cell-walls and forthwith grow into the cells. There are also fungi which, while able to pierce the outer cell-walls, and grow forward in the thickness of the wall itself, cannot enter the living cells themselves--_e.g._ _Botrytis_. In the example mentioned, the fungus excretes a poison, oxalic acid, which soaks into and kills the cells next its point of attack: into these dead cells it then extends, and, invigorated by feeding on them, extends into other cell-walls and excretes more poison, and so on. On the basis of the foregoing it seems possible to sketch a general view of the nature of parasitism. In order that a fungus may enter the cells it must be able to overcome not only the resistance of the cell-walls, but that of the living protoplasm also: if it cannot do the latter it must remain outside, as a mere epiphyte, or at most an intercellular endophyte. If it can do neither it must either content itself with a saprophytic existence or fail, so far as that particular host-plant is concerned. Its inability to enter may be due to there being no chemotropic attraction, or to its incapacity to dissolve the cell-walls, or to the existence in the cell of some antagonistic substance which neutralises its acid secretions, destroys its enzymes or poisons, or is even directly poisonous to it. Moreover when once inside it does not follow that it can kill the cell. The protoplasm of the latter may have been unable to prevent the fungus enemy from breaking through its first line of defence--the cell-wall, but it may be quite capable of maintaining the fight at close quarters, and we see signs of the progress of the struggle in hypertrophy, accumulation of stores, and other changes in the invaded cells and their contents. Finally, the invested or invaded cell may so adapt itself to the demands of the invader that a sort of arrangement is arrived at by which life in common--_Symbiosis_--is established, each organism doing something for the other and each taking something from the other. In this latter case, which is often realised--_e.g._ lichens, leguminous plants and the organisms in their root-nodules, mycorrhiza, etc.--we leave the domain of disease, which supervenes indeed if the other symbiont is lacking.