306 MORPHOLOGY OF THE ASPERGILLACE^E. 



In the differentiation of species, the first point to consider is 

 the colour of the herbage (young growth exclusively !) ; then the 

 size and build of the conidiophores and conidia, and, finally, the 

 physiological characteristics, such as food requirements, optimum 

 temperature, energy of growth, special influences, &c. Moreover, 

 the influence (if any) of the substratum sugar, albumin, and 

 also gelatin must be observed in each case. Differences that 

 have not yet been fully appreciated also exist in the behaviour 

 toward gelatin, and in the production of colouring-matters in the 

 mycelium or nutrient solution, &c. Attempts to identify old 

 vegetative growths from the colour of the conidia, which is liable 

 to a speedy change, especially in green species, are not to be 

 recommended, the preparation of a young culture being essential, 

 and other characteristics are liable to alteration as the cultures 

 become aged. Many of the old soi-disant species undoubtedly 

 owe their alleged existence to insufficient appreciation of these 

 circumstances. 



The conidiophore can in many cases be identified by the un- 

 aided eye, since it measures about 1-2 mm. in height (A. niger, 

 A. glaucus, A. oryzce, A. clavatus, A. candidus, &c.). Sometimes, 

 under favourable conditions of growth, the length is nearly 

 doubled (A. Wentii, A. ochraceus, &c.), whilst in adverse circum- 

 stances it may be considerabty less (0.5-0.25 mm.). Such dwarf 

 conidiophores are of common occurrence in otherwise luxuriant 

 species (A. oryzce, A. candidus, A. glaucus), accompanied by 

 morphological modifications. Only a single species, A. giganteus, 

 far exceeds the average height in its mucor-like conidial vege- 

 tation, the slender conidiophores averaging 1-2 cm. in length. 

 Numerous species are characterised by the constant formation of 

 small and very small conidiophores (A . fumigatus, A. nidulans, 

 A. minimus, A. Eehmii, A. spurius, A. flavus), which cannot be 

 detected as such by the unaided eye, except under favourable 

 conditions, their length averaging less than i mm., and occa- 

 sionally falling below 0.5 mm. (A. fumigatus, A. minimus, A. 

 Rehmii), or even as low as o.i mm. (A . fumigatus), so that the 

 nearly smooth surface growth closely resembles Peuicillium. 



The dimensions of the conidia vary between the limits of about 

 3 and 10 p, the latter size being rarely exceeded. Some species 

 invariably produce microspores exclusively, the conidia measuring 

 only about 3/1 in diameter (A. nidulans, A. minimus, A.fumigatus, 

 and frequently A. niger). The other extreme is reached by the 

 species forming macrospores, with conidia measuring at least 

 5-6 p, and often irregular in size (A. glaucus, A.flavus,A. oryzce, 

 A. Tokdau), attaining a diameter of 7-10, and sometimes as 

 much as 15 /*, in the case of A. glaucus and A. ToJcelau. An 

 intermediate position in this respect is occupied by the species 

 (A. candidus, A. clavatus, A. Wentii, A. giganteus, &c.) with 

 conidia measuring about 3.5-5 /*, these being preferably classed 



