CYSTOPUS, OR WHITE RUST. 139 



not become perfectly ripe till the host-plant has decayed. Al- 

 though the dead plant may be reduced to tinder by drought and 

 frost, the oospores or resting spores remain alive and uninjured. 



The oospores germinate on the ground during wet weather in 

 spring, and according to observations made by numerous micro- 

 scopists, this is what occurs : — The brown exospore bursts irregu- 

 larly, and the colourless endospore protrudes in a short, thick 

 tube, which gradually becomes a large vesicle, into which the 

 protoplasmic contents of the endospore enter. Eventually, the 

 endospore entirely quits the exospore, and the little masses of 

 protoplasm accumulate in a globular mass in its centre. Finally, 

 the spores isolate themselves from each other ; then for some 

 minutes they swarm within the vesicle, which presently gives way, 

 and the zoospores disperse in the surrounding moisture (PI. XV., 

 Figs. 13 — 15). We thus find repeated, after an interval of from six 

 to ten months, the phenomena described as belonging to the 

 chains of conidia. The zoospores produced in both cases are 

 exactly similar, as is also their future career. 



The activity of the zoospores lasts from two to three hours ; 

 then the cilia disappear, and the now immoveable spore assumes a 

 globular form, covers itself with a membrane of cellulose, and 

 puts forth a germinal tube, the extremity of which swells into a 

 small bladder, and receives by degrees the whole of the proto- 

 plasm. 



If the spore has not by this time come in contact with the 

 host-plant, it perishes. But if it does find a suitable host, it 

 attaches itself to, but never enters, one of the stomata; the germ- 

 tube is pushed forth, and at once enters the air-cavity, and with it 

 the entire protoplasm of the spore (PL XV., Figs. 16 — 18). The 

 latter remains as a delicate vesicle, containing a little watery fluid, 

 which soon shrinks and escapes further observation. But here a 

 curious point arises, and as the credit of the observation is due to 

 De Bary, it will be well to quote from his remarks on the subject. 

 He had been making an extended series of experiments on the 

 infection of plants with Cystopiis spores by artificial means, and in 

 giving his results he says : — " If one uses for experiment leaves or 

 stems of plants, which generally bear Cystopiis, the germs enter 

 the stomata quickly, take the form which has just been indicated, 

 but never show any ulterior phenomena of vegetation. If in a few 



