220 GERMINATION OF SPORES 



humidity gradient between the air and the spore membrane is a 

 more decisive factor than relative humidity in influencing germi- 

 nation. In C. fulvum moisture content of the spores is low. He 

 concluded that the higher is the gradient, the better are the 

 chances of germination. 



In many of the studies concerned with germination of rusts, 

 infected host tissues or the spores themselves are floated on water. 

 Blackman (1903) noted that the submerged germ tubes (promy- 

 celium)"of Uromyces fabae, Fncchiia graminis, and Phragnridium 

 ruin grew to considerable length with the protoplasm collected 

 near the apex and that basidiospores were not formed unless the 

 tube reached the air, whereas in moist air the tubes were short and 

 4-celled, and each cell possessed a sterigma upon which a basidio- 

 spore was borne. This morphological modification in type of 

 termination is now known to be related to the fact that rusts 

 forcibly expel their basidiospores, which are adapted for dispersal 

 by air. 



Evidently alternate wetting and drying play an important part 

 in the spore germination of some species. Jahn ( 1905) stated this 

 to be true of certain slime molds. Alternate wetting and drying, 

 he believed, activated the glvcogen-cleaving enzymes, thus caus- 

 ing glycogen in the spore to be converted into maltose with re- 

 sultant increase of osmotic pressure. This explanation may well 

 apply to other kinds of fungi, but it is conceivable that modifica- 

 tion of the spore wall itself may result from alternate wetting and 

 drying and that this change is an important factor in germination. 



Little is known concerning the application of findings from 

 laboratory studies on the relation of moisture to spore germination. 

 A body of data is much needed, especially on the relation of mois- 

 ture to germination and infection by plant pathogens. Observa- 

 tional evidence, which is insufficient and which may indeed be 

 misleading, has led to the conclusion that outbreaks of some plant 

 diseases are caused by dry weather, others by wet weather. 

 Among studies of this kind is that of Jones ( 1923 ), who attempted 

 to correlate the moisture-holding capacity of the soil with germi- 

 nation by Ustilago avenae. She placed chlamvdospores on agar 

 between filter papers and then placed them in soils containing 30, 

 60, or 80° \ of their water-holding capacitv. At favorable temper- 

 atures germination was highest at 30 , slightly less at 60%, and 

 markedly less at 80%. At 80 , which is also unfavorable for in- 



