392 SPORE GERMINATION 



62). Here an unspecified contact stimulus, "thigmotropism," is sup- 

 posed to operate. The possibility of chemotropism has been reopened 

 more recently in studies on Saprolegnia ferax (87). 



One type of germ tube tropism is well documented — the negative 

 tropism of germ tubes toward each other (55, 100, 117). As described 

 by Stadler (263), germ tubes of Rhizopits nigricans form on the side of 

 the spore distant from other spores and, in addition, actually bend away 

 from other germ tubes during their growth. The stimulus is almost 

 certainly chemical, and it is tempting to consider an auxinlike growth 

 factor in explanation of the bending phenomenon. This possibility is 

 also suggested by the finding that germ tubes of rust fungi are longer 

 when germinating spores are crowded than when they are more dis- 

 perse (83,314). However, Stadler (263) presents some evidence against 

 an explanation based on growth factors; instead, he postulates an un- 

 stable inhibitor which has the effect of thickening or strengthening the 

 wall of the spore or germ tube. The last word on this problem has 

 clearly not been spoken. 



Germ tubes of sporidia and of uredospores of Puccinia spp. react 

 negatively to strong light; studies with filters indicate that the active 

 wavelength is in the blue region of the visible spectrum (90, 98, 235, 

 266). It will be recalled from Chapter 1 1 that it is these shorter wave- 

 lengths which are most active in phototropic and inductive effects on 

 reproductive structures. Negative phototropism is also shown by co- 

 nidia of powdery mildew fungi (204, 305). 



3. SPORE METABOLISM 



Water, minerals, carbohydrates, and other constituents of fungus 

 spores have been mentioned incidentally in Chapter 2. In relation to 

 spore germination the important materials are water — considered else- 

 where in this chapter — and, probably, lipids. It should also be recalled 

 that spores of Aspergillus niger are relatively high in total phosphorus, 

 a part of which is in organic combination (7, 154). 



Although the fat content of most spores is not unusually high (248, 

 271), three bits of evidence suggest that fats provide energy for the ger- 

 mination of spores which do not require an exogenous source of car- 

 bon. First, microscopic observation shows that stainable lipids disap- 

 pear during germination (82, 150). Second, the respiratory quotient 

 of Neurospora sitophila macroconidia is about 0.7 (208), a value char- 

 acteristic of, but of course not sufficient evidence for, fat oxidation. 

 Finally, the data of Table 1 show that, at least in high-fat spores, the 

 principal organic material disappearing during germination is the 



