274 DIVISION II,—COURSE OF DEVELOPMENT OF FUNGI. 
UREDINEAE. 
Section LXXIX. The Uredineae, as far as we are acquainted with them, are 
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 branchlets 
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 tremelloid 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 aeczdia, 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 7). 
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 
with 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 narrow 
interstices containing air. This body, which corresponds in all important points to the 
one to which I formerly gave the name of derithecium, 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 A). 
In form it is spherical or flattened vertically. The Aymenium 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, 125) with 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 
