106 



I-'jIArO-FCXGlS. 



directly or indirectly one or more new rudimentary mycelium threada 

 i'ig. 1- ("germ-tubes"), with which the process of de- 



_ ^ N velopmcnt just sketched begins afresh. 1 have 



here employed for the sexual organs and tlieir 

 products the terms whicli at present are most in 

 use, and shall continue to do so in the following 

 pages, though other terms might be employed 

 which are more correct. The use and explauad 

 tion of these, however, would lead me beyon- 

 niy present purpose. 



The known forms of mycelium present great 

 similarity of structure, notwithstanding some pe- 

 culiarities whii-li supply Viiluable cliaracteristics 

 for distinguishing species 



The characters derived from the structure and 

 growth of the conidiophores divide the Perono- 

 sporece generally into two, perhaps better into 

 three, genera, Cystoptis, Peronospora, and a third, 

 which may be called Phytophthora. 



The conidiophores of Ci/stoptcs, the Fungus of 

 the " white rust " of plants, are short, club- 

 shaped, terminal branches of the mycelium, 

 which bear on their expanded apex a single row 

 of conidia, developed in basipetal order. The 

 conidiophores grow in large bunches (see Fig. 

 1). In Peronospora (see Fig. 2) the conidio- 

 phores occur singly, or in small bunches, and are 

 tree-like or dichotomousiy-branchiug tubes. On 

 the sharp end of each one of these branches a 

 solitary conidium is formed, and this is never 

 followed by a second. 

 Phytophthora (see Fig. 3) is distinguislied from Peronospora in 

 liaving not one but several conidia successively formed at the end of 

 each branch of the tree-like conidiophore. When the first conidium 

 is ripe, it is pushed to the side by an imequal swelling of the point to . 

 ■which it is attached. The top of this sA-oUen portion then begins to 

 grow in the original direction of the branch into a new, conical 

 point, and when this has reached a length equal to that of a conidium 

 or a conidium and a half, a new conidium is produced at its apex. 

 The same process may be repeated in vigorous specimens from ten to 

 fifteen times. After the falling of the very easily-shed conidia, as 

 many swellings remain on each branch of the conidiophore as there 

 had been conidia ; such swellings do not occur in the stems of the true 

 Peronosporoi, but are a sure empirical character of the Phytophthora. 

 Among the well-known forms of tlie family, the Potato-fungus, Pero- 

 nospora infestans, Mont., is the only one which has this peculiarity. It 

 may therefore be distinguished as Phytophthora infestans. 



The further development of the mature conidia is the same in 

 Cystopus, Phytophthora, and many species of Peronospora (see Fig. 4). 

 If fresh ripe conidia are placed in water, their protoplasm, separated into 

 three to eight or more portions, issues through the swelling gelatinous 

 membrane of the tops of the conidia, and move freely in water. 



Branch of the 

 mycelium of Cys- 

 topus Porttdacd', 

 Lev., with conidio- 

 phores. Magn. 390 

 diam. 



