136 TEXT-BOOK OF FUNGI 



decoction, sugar, etc., the germ-tubes of fungus spores 

 sown on the surface of the epidermis or mica grew towards 

 and finally passed through the stomata or pierced holes 

 into the nutrient substance below. He also demonstrated 

 that the hyphae of Penicillium glaucum and species of 

 Botrytis passed through the epidermal cells of living 

 leaves, which had previously been charged with a posi- 

 tively chemotactic substance by hypodermic injection. 



Certain substances are also known to be negatively 

 chemotactic, and some repellent, either having no directive 

 influence, or deflecting the germ-tubes from their vicinity. 

 A positively chemotactic substance need not necessarily 

 be a food-stuff. 



The various chemical substances present in the cells of 

 plants render it difficult in most cases to determine which 

 one or more is chemotactic to a given parasite. I used 

 the cell-sap of the cucumber plant for injection into the 

 living leaf of an orchid still attached to the plant. Spores 

 of a fungus parasitic on cucumber Cercospora melonis 

 were sown on the surface of the injected portions of the 

 orchid leaf. In due course a few conidiophores bearing 

 conidia appeared on the injected area of the orchid leaf. 

 These spores were in turn sown on other orchid leaves 

 injected with cucumber plant sap. This mode of procedure 

 was repeated twenty-four times, and the spores of the 

 twenty-fourth generation were found to be capable of 

 entering the tissues of the orchid leaf without having to be 

 coaxed inside by injecting cucumber cell-sap. In this case 

 the parasite on cucumber had been gradually educated to 

 accustom itself to the special chemotactic substance 

 present in orchid leaves. 



In another experiment, Trichothecium candidum, a pure 



