The various genera of these endophytes owe their distinctions to the form, or mode of development of their true spores. In one instance these spores are united in pairs, or divided by a septum, so that they are two-celled: these are named _Puccinia_. In another instance the spores are one-celled, and at first borne upon a stalk or peduncle, from which they are detached in ripening: such are called _Trichobasis_. It is unnecessary here to indicate all the variations to illustrate the fact that the generic distinctions are based upon the characters of the true spores. How unsatisfactory such a mode will appear, when subjected to experience day by day, a botanist would suspect. In the same pustule, resting upon the same cushion of mycelium, the spores of an _Aregma_ will be found with those of a _Lecythea_, and those of a _Puccinia_ with _Trichobasis_. More than this has even been affirmed. The alternation of generations, known to students in the animal world, is here repeated in the vegetable. Dr. de Bary declares that certain data appear to indicate that _Æcidium_ constitutes not a genus by itself, but are organs in the development of some other germs and species, possessing its spermogonia, its _Æcidium_; its _Uredo_, and its spores, properly speaking; whilst in others the _Uredo_-form the _Puccinia_-form, and the _Æcidium_-form may alternate. It is not our intention to enter deeply upon the discussion of this subject, of so little interest to the beginner, and so out of place in an introduction to the study. That forms and conditions are multifarious, and that an entire revision of the classification is inevitable, are facts which do not require many words to establish. Already it is to be feared that in this brief chapter we have said too much, and must recommend its perusal again, when the names and characters of the genera alluded to have been rendered more familiar. It could scarcely have been permitted that the student should go far without being cautioned that there is such a thing as di-morphism in microscopic fungi; and the explanation of such a phenomenon must presuppose a certain amount of knowledge which, thus far, the reader could not have acquired. Hence an anomaly, to escape from which an ultimate return to the subject will be necessary. In a recent account of Dr. de Bary’s experiments,[3] an interesting history is given of the development of a rust-like fungus, which is common on many plants of the pea and bean tribe. As it may serve to illustrate some of the preceding, as well as subsequent, remarks on development, an abstract shall close this chapter. Footnote 3: De Bary—“Annales des Sciences Naturelles,” ser. 4, vol. xx.