284 



GENERAL MORPHOLOGY OF PLANTS 



439. 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 gen- 

 era). 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 fungus. In the onion mildew (Peronospora schleid- 

 eniana) the spore germinates by a germ tube which forms the 

 mycelium. In the grape downy mildew (Plasmopara viticola) 

 the protoplasm of the spore (conidium) first divides into several 

 smaller bodies which form bean-shaped zoospores with two 

 lateral cilia, as in Cystopus (see Fig. 230, B). These escape from 

 the conidium (really a spore case in the downy mildews), swim 

 about for a time, then come to rest, germinate and produce 



Fig. 228. 



Fertilization in Peronospora alsinearum; tube from antheridium carrying in the sperm 

 nucleus in figure at the left, female nucleus near; fusion of the two nuclei shown in the two 

 other figures. (After Berlese.) 



mycelium again if they are favorably situated. The spores of 

 the potato blight germinate in both ways. Successive crops of the 

 asexual stage are rapidly formed, and the disease spreads. 



430. Sexual reproduction. This takes place by the forma- 

 tion of the sexual organs, sperm and egg cases (antheridia and 

 oogonia), and the fertilization of the egg. The egg case is a 

 short branch which swells out into a large rounded body. A 

 single egg is formed from the centrally located protoplasm, leav- 

 ing a layer of protoplasm around the outside which does not 



