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, 



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