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A HISTORY OF HERTFORDSHIRE spores that we know one from another, they only being visible on the surface ; the mycelium is hidden in the tissues of the host-plant and consists of long and slender filaments called hypha? which sometimes per- vade the whole of the host-plant except its root. The cycle of changes through which the Uredineas pass is very varied. In the simplest life-cycle there sprouts from the teleutospore a tiny hyaline tube, the promycelium, from each segment of which there arises a short branch ; the distal end of each branch falls off as the promycelial spore, moisture causes it to germinate, and if it should happen to fall upon a living leaf of the host-plant proper to its species the germ-tube enters the tissue of the leaf and gives rise to mycelial hyphae from which teleutospores are developed, usually on the under surface of the leaf. The mycelium does not always directly give rise to teleuto- spores ; frequently it first produces organs called spermogonia, and then the most highly coloured and conspicuous of all kinds of spores, the aecidiospores. These are the true cluster-cups, and at one time they constituted the then important genus flLcidium, but now nearly all the species formerly referred to this genus are known to have been founded on the ascidiospore stage of species belonging to other genera. The aecidiospores may directly produce teleutospores, or firstly uredospores, which in their turn may produce teleutospores or may for some genera- tions reproduce themselves as uredospores, but teleutospores must eventually be formed. Perhaps the most interesting phenomenon in the life-history of the leaf-fungi is the passing in some species of a part of their life on one kind of plant and another part on a different kind. This is called hetercecism, and it was first proved to exist in 1864. More than a century earlier it was generally recognized that the presence of the barberry (Earbarea vu/garis) is injurious to growing crops of wheat and of some other cereals ; but the cause was unknown, the fact often dis- puted, and the remedy therefore often neglected, until it was proved by De Bary that Puccinia graminis, the microscopic fungus which attacks the wheat plants, is a later stage in the life-cycle of JEcidium berberidls^ the cluster-cup of the barberry. As another instance of hetercecism may be mentioned one of the best known of all the cluster-cups, which occurs on the leaves of the lesser celandine (Ranunculus ficarid) and on those of R. repens and R. bulbosus. This has been known until recently as /Ecidium ranunculacearum^ and has also been named M. ficarice, but it is now found to be an early (aecidiospore) stage of Uromyces poce, the mature form of which (the teleutospore) occurs on the grasses Poa trivia/is, P. pratensis, and P. annua. The true Uromyces ficarice is only known to occur on the lesser celandine. Owing to the various forms which the spores assume in their differ- ent stages, and to hetercecism, to which about fifty species are subject, the number of recorded species of Hertfordshire Uredinese has had to be considerably reduced. As an instance of the record of a species under three names may be mentioned the rose-pest Phragmidium subcorticatum, 74