FJiUCTlKICATKJN BY ZVOOSl'OHKS. 19 



of otlifi- living' (jr^aiiisiiis, is tlie lesult of two factors; iulierited 

 jirojjoities ami the sum of external active forces. The sUitement 

 tliiit zygospores are encountered solely in the class of the Zygo- 

 mycetes does not imply that these fungi rely exclusively on the 

 organs in question for their reproduction. On the contmry, 

 this method of reprcxluction is nire, the oi-dinary and most 

 frequent method being hy spomngial or conidial fructification, 

 tis will be more specificjdly shown in Chapter xliii. At 

 present we will consider the influence of exteriml conditions on 

 the occurrence of one or the other methotl of fructification. In 

 the cjise of S/torwliiita r/raiulig, G. Kleus (II.) has shown the 

 jirincipal factor to V)e the proportion of moisture in the surround- 

 ing air. When this approaches the limit of siitunition, zygosjMjres 

 alone are produce<l ; but on the moisture being diminished, 

 .sj^Kjrangia are fonne<l as well ; and finally, the latter are ex- 

 clusively produced when the relative moisture has fallen to 

 about 65 per cent, .so that active tmnspinition can occur. The 

 chemical composition of the nutrient substiiitum is al.so of influ- 

 ence, more particularly in that, in presence of an excess of 

 nitrogenous subsUinces, sporangia alone, and no zygospores, are 

 formed, the last named requu-ing the presence of suitible curbo- 

 liydnites for their production. It is therefore particularly 

 interesting to observe how decisively this fungus discriminates 

 for this purpose between individual isomeric sul)st:inces. Thus 

 zygospores are produced when the available carbohydrate consists 

 of manite or dvdcite (C,.H,^0,.,), dextrose, levulose, galactose 

 (C^H,.,Og), .sjiccharo.se, maltose (C,.,H.„0,j), or dextrin; whereas 

 sponingia alone are produced when the nutrient substnitum 

 contains one of the following carbohydrates: sorbite (CgHj^O^), 

 sorbinose (CgII,„0^;), lactose (C,.,H.,.,0,,), niffino.se. isodulcite or 

 erythrite. It hius also been fountl that conditions adverse t<i the 

 formation of zygospores favour that of azygospores. 



According to observations communicated privately by E. L'h. 

 Ifansen, the pro<luction of zygospores in ^jxiroilinia (/raiults {us 

 well as in a hitherto unde.scribed species of Mu<'or) is not .so 

 strictly dependent on the fulfilment of such conditions, but 

 occurs readily and without any special experiment;tl preparation. 

 On the other hand, it cannot, .so far, be induced at all in the case 

 of several other species of Zi/i/omi/retts. 



i 222.— Fructification by Conidia. 



The .nature of this fructification and of its fundameutiil 

 chanicteristic of ditYerentiation from spomngial fructi'" 1 



lies in the circum.^^tance that the spores, in.^tead of be _ 

 closed in a cell, separate by constriction from the fruit-bearing 

 stem (conidiophore) externally, and are therefore known as 

 exospores. In so far as the behaviour of the conidiophore is 



