CHAPTER XXXIII 

 UREDINALES 



The Uredinales, frequently called Uredineae or rusts, form a parasitic 

 branch of the Auriculariales; as in the latter, the zeugite develops to a 

 special organ which is encysted and acts as a resting spore in the higher 

 forms. They differ from the Auriculariales, however, by the higher 

 differentiation of these resting spores, by the varied types of other spore 

 forms and by the lack of fructifications. They include three thousand 

 species which are exclusively parasitic on cormophytes. Saprophtyic 

 forms or forms cultivatable on artificial media, are still unknown. 



The mycelium consists of regularly septate, ramose hyphae, whose 

 cells, particularly when young, contain numerous orange-colored drops 

 of oil. Normally it grows intercellularly and sends allantoid, seldom 

 branched or knotted, haustoria into the host cells which are not killed 

 but only robbed of part of their substance. In many species, the myce- 

 lium is limited to a small area near the place of infection; these species 

 are annual; in others, outside of the tropics, it penetrates the whole 

 host or a large part of it, overwinters in the perennial parts and grows 

 again in the following spring. The mycelium of these species is perennial; 

 it produces many kinds of malformations, sparse growth of the whole 

 plant, enlargement or reduction of leaf surface, suppression or stunting 

 of the floral parts, witches' brooms, etc. On Urtica parviflora, the 

 hypertrophies which are produced by Puccinia Caricis accumulate in 

 such large quantities that in central Asia they are eaten by the natives. 

 In one case, in the Javan Goplana mirabilis on the leaves of Meliosma, 

 extramatrical mycelium has been demonstrated. The cells of the hyphae 

 are uninucleate in some portions of the life cycle, in others wholly binu- 

 cleate; the haustoria are at times four nucleate. Occasionally clamp 

 connections appear on the hyphae, as in the Auriculariaceae (Voss, 1903). 



There are five spore forms: pycnospores, aeciospores, urediniospores, 

 teliospores and basidiospores. We shall discuss them in the order they 

 usually appear in nature. 



The first spore forms visible in spring are the pycnospores (also 

 known as pycnidiospores, or spermatia). They arise exclusively in 

 uninucleate mycelium and in certain sori, pycnia (pycnidia or sper- 

 mogonia). These lie either between the epidermis and cuticle, and are 

 then more or less flat (Fig. 371, 2), or they are hypodermic, and are then 

 often sunk into the host tissue as spherical or perithecial-like structures. 



553 



