THE GENUS ASPERGILLUS. 



301 



relationship. With regard to the genus AspergiUus, the reader is 

 also referred to the works of WILHELM (I.), SIEBENMANN (I.), 

 TIRABOSCHI (I.), and WEHMER (XVII.), as well as to the recent 

 publications of the French authors, dealing with pathogenic fungi 

 and cited in connection with A.fumigatus (p. 316, vol. ii.). 



The conidiophore, which, in the mature condition, is far 

 broader and has thicker walls than the vegetative hyphse, is 

 mostly unbranched and aseptate, springing from a vertical hypha 

 with globular terminal swelling. As a rule, it is clearly separable 

 into stem and globule (see Fig. 163), the last-named being covered 

 all over or on the top with a large number of closely set 

 sterigmata of variable length and shape and producing conidia, 

 either direct, or after the formation of secondary sterigmata. 

 The conidia are globular or ellipsoidal, always unicellular, with 

 delicate smooth or finely granular walls, and grow in long, 

 wreathed chains, and in addition to covering the heads with a 

 loosely coherent dust (usually coloured), also impart to the 

 herbage of the mould its specific colour (green, blackish brown, 

 yellow-brown, yellow, etc.). The globule, which is not morpho- 

 logically constant for the species, and may be spherical, oval or 

 elongated, in which latter case it does not exhibit any sharp line 

 of demarcation from the stem, which it also resembles generally 

 in being colourless, tough-skinned, and occasionally very brittle 

 (A. minimus). In the microscopical examination of the head 

 (freed from conidia and lightened in the course of preparation) 

 the most important features are the relative length (in comparison 

 with the globule), and more especially the radial (A. niger) or 

 upright (A. fmniyatus') position of the sterigmata. The number 

 of ultimately developing secondary sterigmata (or sterigmata of 

 any order in comparison with the supporting basidia) varies from 

 2 to 12 according to the species and other circumstances, 

 and these are all considerably more delicate and shorter than the 

 primary forms. The form of the conidia and the character of 

 the membrane (smooth or rough) may vary in the same species 

 (though chiefly through the influence of the medium or of age), 

 and their dimensions sometimes differ considerably (A. Tokelau, 

 A. oryzce, A. Jlavus), even in those from the same head, probably 

 as the result of growth subsequent to constriction. In other 

 cases, however, great regularity is observable on these points 

 (A. niger, A. davatus), so that in many instances the dimensions 

 afford a reliable diagnosis. In view of the variable dimensions of 

 the conidiopores noted in one and the same culture, apart from 

 differences in nutrition and temperature, the value of accurate 

 microscropical measurements is after all merely relative, though 

 they cannot be entirely dispensed with and are even capable of 

 affording valuable indications when intelligently applied. While 

 scarcely necessary for the mere differentiation of dwarf and 

 normal growths, the measurements ascertained, nevertheless, 



