126 FIRST GROUP. THALLOPHYTES. 



were produced by hyphae. The gonidia in the cases cited, that is the cells of the 

 nostoc-chains which supply the gonidia, increase in size, alter their form, and become 

 invested with a thick cell-wall, such as is not observed in cells not pierced by the 

 hyphae ; finally they lose their colour and disappear. In other cases too it would seem 

 that gonidial cells are really destroyed by the Fungus. 



But the gonidia in their turn exercise an influence on the hyphae ; if a hypha comes 

 into contact with an Alga, it is excited by it to vigorous growth, as shown by a rapid 

 increase in the number of its cells and the formation of numerous branches, which 

 grow round the Alga (Fig. 84). An analogous growth will be described further on in 

 discussing the peculiar symbiosis of certain Algae and Hepaticae (see p. 23). It is remark- 

 able that in some Lichens two different Algae are found serving as gonidia in the same 

 Lichen-thallus. The Algae which supply gonidia are all widely disseminated ; Proto- 

 coccus^Nostoc, Scytonema^ and their allies are found almost everywhere on the trunks of 

 trees, on stones, on the ground and elsewhere. This explains the great prevalence of 

 lichens, as well as the fact that they usually are the first forms of organic life that appear 

 on the fresh surfaces of rock or other material. On this point Bornet's writings should 

 be consulted. 



The genera Cora and Khipidonema, which have been already mentioned, are 

 distinguished from all other Lichens, according to Mattirolo 1 , by the fact that the 

 Fungus in them is not an Ascomycete but a Basidiomycete, and other cases of the kind 

 will probably be discovered. The genus Cora is a tropical lichen living on trees, with 

 the surface of its membranous thallus marked in concentric zones ; its gonidia belong 

 to the genus Chroococcus, those of Rhipidonema to Scytonema ; the hymenium is on the 

 under side of the thallus and consists of basidia producing each one spore. The habit 

 of these lichens places them nearest the basidiomycetous genus Stereum. It is 

 obvious that the existence of these forms is a new and interesting proof of the 

 correctness of the view here taken of the origin of the Lichen-thallus. 



5. THE UREDINEAE (AECIDIOMYCETES) 2 . 



If we confine our attention in this as in the preceding groups to the forms whose 

 development is thoroughly known to us, we find in respect to the phenomena of 

 propagation two extreme cases ; in the simplest case the mycelium bears a fructifica- 

 tion corresponding to that of the Ascomycetes and known as an aecidium, which in 

 its mature state consists of a cup-shaped envelope, the peHdium, and a hymenium 

 occupying the bottom of the cup, from the basidia of which spores are obtained one 

 after another by abscision (Fig. 85). The spores thus formed (aecidiospores] 

 germinate immediately and produce a short filament consisting of a few cells, which 



1 Contribuzioni allo studio del genere Cora Fr. Nuovo giornale Italiano Botanico, Vol. XIII, 

 No. 4. Ottob. 1881 ; [see also Biol. Centralblatt, I. Feb. 1882. Johow, Die Gruppe d. Hymeno- 

 lichenen (Pringsh. Jahrb. XV. 1884).] 



2 Tulasne, Ann. d. sc. nat. 3 serie, T. VII, and 4 serie, T. II. De Bary, Unters. ii. d. Brand- 

 pilze, Berlin 1853. De Bary, in Ann. d.sc. nat. 4" serie, T. XX, und Monatsber. d. Berl. Ak. 1865. 

 Reess, Die Rostpilzformen d. deutschen Coniferen, Halle 1869 (Abh. d. naturf. Ges. Bd. XI). 

 Winter, Die Pilze (Rabenhorst's Kryptogamenflora, II. Aufl.) ; Id., Zusammeristellung d. ii. d. 

 Wirthwechsel d. verschied. Formen bekannten. De Bary, Aecidium abietinum (Bot. Zeit. 1881). 

 [Id., Vergl. Morph. u. Biol. d. Pilze, p. 308 (literature). Plowright, Life-history of Aecidium Belli- 

 dis, DC. (Journ. Linn. Soc. vol. XX). Id., Mahonia aquifolitim as a nurseof the wheat mildew (Puce, 

 graminis}, and On the life-history of the dock Aecidium (Ace. Rumicis, Schlecht) (Proc. Roy. Soc. 

 Lend. vol. xxxvi) ; also On the life-history of certain British heteroecismal Uredines (Q. J. M. S., 

 Jan. 1885).] 



