Plate VI. represents the plant living at a greater depth with less air, expending, by means of sugar as source of heat, the energy which it acquired in vegetating under the influence of the oxygen of the air. The filaments are fewer and older in aspect, and the number of cellular forms is proportionately larger than in the former case, the budding giving rise by preference to spherical or oval cells. On a single cell we often see two, three, four, five, six, and even more buds. When the buds of the oval or spherical cells detach themselves whilst young, they often resemble in form and size cells of ordinary yeast, nor can even considerable experience in this kind of observation always enable us to distinguish them. Hence we may easily understand how many have come to believe, with so skilful a botanist as Dr. Bail, in the transformation of _mucor_ into yeast. With the forms represented in Plates V. and VI., the plant is more of a ferment than of a fungoid growth. In such cases the weight of sugar decomposed in comparison with the weight of new cell-globules formed is very considerable, an effect which is more marked the less air the plant has at its disposal. Under the latter conditions, however, vegetation is slow and laborious, and the ferment very soon assumes an aspect of age, and we must constantly rejuvenate the cells by bringing them into contact with oxygen, and subjecting them to the action of limited quantities of that gas, and so promote their vegetation and prolong their fermentative activity. This effect we brought about when we retransferred the liquid and its deposit of _mucor_ from the test-glass to the flask, thus bringing them into contact with fresh air. We saw cells that appeared old, dark, and highly granulated, become inflated, grow more transparent, and fill with a gelatinous protoplasm, the few granulations which they still exhibited assuming a brilliant appearance when we succeeded in distinguishing them; and finally, a very active budding was set up. Under this reviving influence life could continue once more away from the air, although with difficulty, so that fermentation would be most intense if the large filaments and their conidia were constantly being removed from and to the action of air. The preceding plates show several instances of this rejuvenescence of the old cells of _mucor_ ferment. [Illustration: Plate 6. Ferment of Mucor.] We have omitted to represent amongst the old cells some cells which have their granulations collected about the centre, with an empty space between the granulations and the exterior borders.[80] In this state, cells are generally dead and incapable of any revival. It is impossible to avoid being impressed by the striking analogies which exist between all these facts and those presented by cells of yeast.