TLUJSTEATIONS OF PHYTOPHTHORA INFESTANS. 423 



affects the germination by influencing the amount of oxygen 

 available for any one conidium. I am satisfied that it is easier 

 to obtain zoospores in dewdrops on the living leaf than in 

 water on glass. Nevertheless, it must not be overlooked that 

 the zoospores will develope on a wet leaf during the nighty in- 

 cluding the early hours of the morning. There can be little 

 doubt that an interesting field of investigation in comparative 

 physiology is here open. 



As to the significance of the secondary conidia and their 

 formation, I have found them empty, as if they had developed 

 zoospores which had escaped ; but have never seen zoospores 

 come from them. 1 have also seen a secondary conidium with 

 what looked like a germinal hypha developed from it ; but this 

 died before developing very far, so that it was impossible to say 

 whether a tertiary conidium could be developed. I have not 

 seen these tubes enter a stoma. The secondary conidium 

 would seem to be a second attempt on the part of the zoospore- 

 producing protoplasm to prepare for the development of zoo- 

 spores again ; there is a loss of substance from respiration, and 

 the cellulose of the hypha has to be formed, and the energy 

 expended is no doubt evolved at the expense of materials in the 

 protoplasm, and not replaced by nutrition. 



Fig. 9. — This drawing was from a preparation of the epi- 

 dermis of an internode on which several of the crowded 

 conidia had germinated as above. The drawing is to scale. 

 As usually happens in such cases as this, the germinal hypha 

 is shorter than those developed in hanging drops, and the 

 secondary conidium oblique. 



Fig. 10. — The preparation shows hyphse of the parasite in 

 the cortical tissues of the internode three days after infection. 

 The chief point of interest is the course of the hyphae, they 

 run between the thin-walled, closely-fitting cells, in the middle 

 lamellse, and even push aside the other layers of the cellulose 

 walls; the diameter of the hypha is considerably greater than 

 that of the walls they traverse. The same thing occurs in the 

 tuber (see fig. 15, belowj. The hyphae branch often; they are 

 devoid of septa and have very thin walls and abundant finely 



