FUNGI, DOWNY MILDEWS 



257 



blight, early potato blight,* etc. During 1849 the potato blight 

 caused almost the complete loss of the potato crop in Ireland, and 

 a serious famine resulted. 



415. The mycelium. The mycelium attacks the leaves, 

 stems, and fruit. The germ tube from a spore enters at a stomate 

 of the leaf or between 



epidermal cells. The 

 mycelium grows be- 

 tween the cells in the 

 intercellular spaces, 

 and is thus said to 

 be intercellular. The 

 mycelium is con- 

 tinuous and multinu- 

 cleate. It develops 

 short special branch- 

 es, of different form 

 in different species, 

 which penetrate the 

 cells and take food 

 from the protoplasm 

 (fig. 216). This kills 

 the cells, and dead 



spots appear on the leaves, fruit, and stems, or the death of leaves, 

 fruit, and stems is the result in some cases. 



416. Asexual reproduction. In asexual reproduction 

 branches arise from the intercellular mycelium, which issue 

 through the stomates, often several together. Outside of the 

 leaf these usually branch (in a different manner in different genera). 

 These are the sporophores or conidiophores. The tips of the 

 branches bear oval spores (conidia). When these fall away they 

 germinate, the manner of germination depending on the genus of 



* The remedy for these blights is to spray the plants with Bordeaux mix- 

 ture before they become infested. Spray when the leaves are young, and 

 then at intervals of two to three weeks. In the case of the grape vine the 

 first spray should be applied .before the buds burst. 



Fig. 216. 



Intercellular mycelium with haustoria entering the cells! 

 A, of Cystopus candidus (white rust); B, of Peronospora 

 calotheca. (De Bary.) 



