Aecidia and Aecidiospores. 17 



The development of the aecidia has been traced by De Bary, Neumann 1 , 

 and others. The hyphae derived from the promycelial spores form at 

 certain points,, deep down in the parenchyma of the leaf, little compact 

 bodies which have been called primordia by De Bary, because they are 

 the beginnings of the aecidia. These bodies gradually increase in size 

 by the rapid multiplication of the mycelium until they assume a spherical 

 form, only being slightly flattened at the part immediately beneath the 

 epidermis, and considerably thickened at the base. If sections aie made at 

 this stage the differentiation of the cells is seen to have begun. From the 

 dense basal layer of hyphae arise numerous, closely-crowded, short, erect 

 hyphae, somewhat club-shaped, and generally known as basidia. From 

 each of these erect hyphae an apical cell is separated off, and beneath that 

 another, and so on until a linear series is formed. At the same time, as the 

 basal cells give rise to the so-called basidia, the surrounding cells develop 

 into the external envelope or peridium. At first they are absolutely indis- 

 tinguishable from the others, but after the third or fourth division they are 

 seen to be much broader. Then, simultaneously with the formation of the 

 first spores, they acquire their characteristic markings and polygonal form. 

 At first the peridial cells are filled with the coloured protoplasm or endo- 

 chrome, but the colour gradually disappears. 



With regard to the peridial cells, Fischer 1 has shown that in 

 Gymno sporangium the sculpturing of the walls may be used for the dis- 

 tinction of species, and while in the genera Puccinia and Uromyces these 

 cells are much more uniform in their markings, yet there is considerable 

 variation. Mayus 1 has examined peridial cells from several species of 

 these two genera, and has found that within the limits of the same species 

 the nature of the peridium undergoes variation under the influence of 

 external conditions. These variations chiefly consist in the relations of the 

 lumen and the thickness of the wall, the lumen being relatively larger in 

 shady places and the converse in sunny places. 



The typical aecidium possesses an external envelope of cells or peri- 

 dium, which surrounds the spores, and when this becomes ruptured at the 

 top to allow the escape of the spores it assumes the form of a cup. But 

 the envelope may assume different forms, or even be absent, and then 

 different names are given to it for convenience, although in its essential 

 character of spore-formation it is still the same structure. When the peri- 

 dium is elongated, and often horn-like, it is called a Roestelia, or it may 

 be rather irregular in shape and confined to coniferous plants, when it is 

 called a Peridermium (not represented in Australia), and when the peridium 

 is absent it is known as a Caeoma, as in C. apocyni. 



Even in the genus Puccinia the pseudo-peridium is wanting in indi- 

 vidual species. In a number of species, as in Puccinia prenanthis, the 

 aecidium is sunken and formed by the more or less altered tissue of the 

 host-plant, and a special pseudo-peridium is either completely wanting or 

 very imperfect. Such aecidia are to be regarded as intermediate forms 

 between the true distinctly walled aecidia and the caeoma form. 



The origin of the aecidium has given rise to a good deal of discussion, 

 and it is not universally accepted that it originates from non-sexual cells. 

 Massee 1 , in his paper On the presence of Sexual Organs in Aecidium. 

 considers it to be a sexual product, and has even drawn the swollen ends 

 of two mycelial hyphae imbedded in the tissues of the host-plant, supposed 

 to be in the act of conjugation. Arthur, in his Problems in the Study of 

 Plant Rusts,' 2 ' and The Aecidium as a Device to restore Vigour to the 

 Fungus 1 comes to the conclusion \that " the aecidium, with its accom- 

 panying spermogonia, represents the original sexual stage of the fungus, 

 and that it still retains much of its invigorating power.'' 



