2 2 British Uredinecs and Ustilao;inece. 



A section through one of them shows it to consist of an 

 external envelope of cells, which encloses the body on all 

 sides, above, below, and laterally. The base of the body 

 is a circular disc (hymenium), which bears upon its upper 

 surface a number of closely packed, erect hyphse (basidia), 

 each of which supports a linear series of spores. The 

 hymenium increases in width as the eecidium develops, 

 but whether from new basidia arising between those at 

 first formed, or circumferentially, is not known. Sur- 

 rounding the circumference of the hymenium is a circle of 

 sterile cells (the peridial cells ; Plate II. Fig. 2), Spore- 

 formation in the aecidium proceeds from above downwards, 

 in the following manner. The cells destined to become 

 spores are enclosed in a hyaline tube (the mother-cell) ; 

 they are at first colourless, but soon within each, from above 

 downwards, a number of granules appear, which, becoming 

 invested with a delicate membrane, rapidly augment in 

 size, from an increase of the enclosed protoplasm, until they 

 touch the walls of the mother-cell. The granular contents 

 are by this time orange {^cidiutJi), or brown {Rcestelia), 

 or remain white in a few species, such as yEc. rumicis, 

 vincce, etc.* 



The growth of the cell continues until it is indistinguish- 

 able from the mother-cell. By the mutual pressure of the 

 neighbouring cells the young spores become polygonal. 

 As this process begins above and continues downwards, or, 

 as it is now termed, in a basipetal manner, it follows that 

 the ripe spores are uppermost. As soon as they have 

 attained their full maturity, they separate from one another 

 and are blown away. In some species {Rcestelia, aecidio- 



* Mr. G. Massee considers the whole ^cidium to be a sexual product 

 resulting from the conjugation of two dilated mycelial hyphse in the tissues 

 of the host-plant {Annals of Botany, June, 1888, vol. ii. pp. 47-5i> pl^^e iv. 

 figs. 1-7). 



