Uarly Blight of the Potato. 465 



are closely examined they are found to be usually marked by con- 

 centric rings, whicli indicate tlie stages of growth in tlie spreading of 

 the spot. Ultim;itely the leaves become Avholly brown on their upper 

 surface, although tlie under surface retains more or less of a green 

 tinge. The stalks are also of a sickly yellow, instead of a bright, 

 healthy green, and latterly lose their succulence, becoming dry. 



The Fungus under the Microscope- 



If one of the spots is examined under tlie microscope it is found 

 to be permeated by fungus filaments, which cause the decay of the 

 tissue, and from these, dark-brown erect or ascending filaments wdth 

 ti-ansverse partitions project and bear the reproductive bodies. The 

 filaments which ascend into the air are called conidio])hores or conidia 

 bearers, because they give rise to conidia at their free ends, which are 

 slightly cup-shaped so as to form a resting place for the conidia. 

 (Fig. :j). These brown conidia are the reproductive bodies, and each 

 consists of a number of compartments divided lengthwise and cross- 

 wise and terminates in an elongated, slender, colourless, tapering- 

 beak with cross partitions, the beak being single, double, or treble. 

 (Figs. 4, 5, and 6). 



When ripe these conidia are easily detached, and under the 

 influence of moisture soon begin to germinate. They do this in a 

 variety of ways, the great difference being in the more or less direct 

 and rapid way in Avhich they reproduce themselves. 



The conidium may directly produce on its beak a short conidio- 

 phore (Fig. 7) which in turn bears a conidium, the one shown in 

 Fig. 4 having fallen from it, or various lateral conidiophores may be 

 formed, giving rise to secondary conidia (Figs. 10 and 11). The 

 terminal beak may produce a germ-tube, which bears a conidophore 

 and secondary conidium. (Fig. 12). All these methods shew how 

 rapidly the conidia may be multiplied without the intervention of 

 much of a vegetative stage. But under normal conditions when a 

 conidium falls on a moist spot on a leaf, various cells may put forth 

 delicate filaments, which enter the stomata or breathing pores and 

 thus produce the mass of filaments from which conidiophores arise, as 

 in Fig. o. 



It can easily be seen how the fungus spreads, and on the dewy 

 leaf reproduces itself with amazing rapidity. 



An extensive series of cultures on potato leaves, in water and in 

 decoction of potato leaves, was carried out by my assistant, Mr. 

 Robinson, in order to observe the methods of spore formation and 

 germination. The diseased leaves received at the laboratory as a rule 

 showed only a limited number of conidia. On placing small diseased 

 spots in petri dishes and keeping thein moist, in twenty-four hours a 

 profuse development of conidia was noted, and in the many hundreds 

 of such cases subjected to the most careful microscopical examination, 

 no single instance of the production of conidia in chains was observed. 

 But in old cultures in potato leaf decoction, a few chains of conidia 



