274 DIVISION II. — COURSE OF DEVELOPMENT OF FUNGI. 



UREDINEAE. 



Section LXXIX. The Uredineae, as far as we are acquainted with them, 



all parasites on living Phanerogams and Ferns. Many species are able to complete 

 their development on one host, as will be described more at length in section CX ; 

 others are obliged to migrate from one host to another in order to arrive at certain 

 stages of their development. 



The mycelium consists of delicate copiously branched septate hyphae, and the 

 cells thus formed, especially when they are young, contain a large quantity of orange- 

 coloured oil-drops. The mycelium spreads chiefly in the intercellular spaces of the 

 parenchyma of the host, but often sends into the interior of the cells short branchleis 

 which themselves have usually short branches, and are then considered to be haustoria 

 (see section V). In Hemileia only haustoria were found by Ward which have the 

 form of unbranched vesicles on slender stalks, not unlike those of Cystopus. We are 

 at present able to divide the Uredineae into two groups according to the course 

 of their development, aecidia-forming Uredineae and the trernelloid Uredineae. 



The development in the former group, which will be first considered separately 

 from the other, agrees so nearly with that of the typical Ascomycetes, that certain 

 stages in each group may be regarded as homologous with one another, though 

 it must be allowed that the proof of the homologies is not quite perfect. 



The sporocarps are termed accidia, and are developed in the subepidermal 

 parenchyma of the host; the mode of development, except in one case which will be 

 noticed in the sequel, is as follows (see Fig. 124 /). 



The first beginnings, primordia, are found in the intercellular spaces of the 

 parenchyma of the host and are composed of a dense weft of felted mycelial hyphae 

 villi air-containing interstices, at first scarcely larger than a cell of the parenchyma, 

 but gradually increasing in size and displacing the surrounding elements of the 

 tissue. The separate cells of the originally narrow cylindrical hyphae of the pri- 

 mordium increase in circumference, the change proceeding from the centre to the 

 periphery of the primordium, and it gradually acquires the appearance of a body of 

 pseudo-parenchyma with round or ellipsoid thin-walled pellucid cells, and 11,1 

 interstices containing air. This body, which corresponds in all important points to the 

 one to which I formerly gave the name of perilhccium, continues to be enclosed in a 

 weft of ordinary mycelial filaments, which are directly continuous with the elements 

 composing its exterior surface. It lies with its apex just beneath the epidermis 

 of its host, while its base reaches some way down into the parenchyma (Fig. 124 . 1 «. 

 In form it is spherical or flattened vertically. The kymenium now makes its 

 appearance at the base of the aecidium, on the flat surface which abuts on the 

 surrounding mycelium ; it is composed sometimes of an irregularly shaped, but much 

 more usually of a circular and continuous layer of short cylindrically club-shaped 

 basidia directed vertically towards the apex ; each basidium abjoints a single long row 

 of spores in basipetal succession one after another (Figs. 124 a, 12,-j widi temporary 

 intermediate cells (page 71). The surface-diameter of this layer and the number 

 of its basidia are at first comparatively small, and some time is required before 

 the ultimate breadth of the mature hymenium is reached; whether the new basidia 

 are inserted between those which are first formed, or grow up outside them, has not 



