The vegetative system of the “rust,” and similar fungi, consists of a number of delicate, simple, or branched threads, often intertwining and anastomosing, or uniting one to the other by means of lateral branchlets. These threads, termed the mycelium, penetrate the intercellular spaces, and insinuate themselves in a complete network, amongst the cells of which the leaf, or other diseased portion of the plant, is composed. High powers of the microscope, and equally high powers of patience and perseverance, are necessary to make out this part of the structure. We may regard the whole mycelium of one pustule, or spore-spot, as the vegetative system of one fungal plant. At first this mycelium might have originated in a number of individuals, which afterwards became confluent and combined into one for the production of fruit, that is to say, an indefinite number of points in the vicinity of the future mycelium developed threads; and these, in the process of growth, interlaced each other, and ultimately, by means of transverse processes, became united into one vegetative system, in which the individuality of each of the elementary threads became absorbed, and by one combined effort a spore-spot, or cluster of fruit, was produced. In the first instance a number of minute, transparent, colourless cellules are developed from the mycelium: these enlarge, become filled with an orange-coloured endochrome, and appear beneath the cuticle of the leaf as yellowish spots. As a consequence of this increase in bulk, the cuticle becomes distended in the form of a pustule over the yellow cellules, and at length, unable longer to withstand the pressure from beneath, ruptures in irregular, more or less elongated fissures (Plate VII. fig. 141), and the yellow bodies, now termed _spores_ (whether correctly so, we do not at present inquire), break from their short pedicels and escape, to the naked eye presenting the appearance of an orange or rust-coloured powder. In this stage the spores are globose, or nearly so, and consist of but one cell Plate VII. figs. 142, 144). It will afford much instructive amusement to examine one of these ruptured pustules as an opaque object under a low power, and afterwards the spores may be viewed with a higher power as a transparent object. The difference in depth of tint, the nearly colourless and smaller immature spores, and the tendency in some of the fully matured ones to elongate, are all facts worthy of notice, as will be seen hereafter.