220 RESEARCHES ON FUNGI 



by continuous observation with the microscope, the apices of the 

 conidia do not keep the same relative distances from one another as 

 they have when they can first be seen, but often alter them consider- 

 ably. This is correlated with the fact that the conidia during their 

 growth in length become curved, so that they intermingle irregularly. 

 Thus, in the end, the mature conidia, collectively, may be tortuous and 

 somewhat spreading or, as frequently happens, they may be compact 

 and intertwining so that they resemble a besom (cf. Fig. 107, F). 



As soon as they have attained their full length and whilst they 

 are still seated on the promycelium, the primary conidia conjugate 

 in pairs. This is accomplished by a bridging hypha ^hich is formed 

 across an air-gap which may be 1-3 times the diameter of each 

 conidium. Exactly how this bridging hypha comes into existence 

 awaits further investigation ; but, in all probability, it is by means 

 of a peg-to-peg fusion, 1 i.e. by two very short lateral branch-hyphae 

 (pegs), one produced by one conidium and the other by the other 

 conidium, coming into existence opposite to one another, growing 

 toward one another through the air, meeting, and fusing at their 

 ends (cf. Fig. 16, p. 32). If the bridging hypha is formed in this 

 way, we have exhibited during its formation a zygotropic phenomenon 

 comparable with that observed by Blakeslee 2 during the aerial con- 

 jugation of the zygophores in some species of the Mucorineae. The 

 bridging hypha is usually situated near the middle of the two 

 conidia involved (cf. Fig. 107, F), but it may be formed at their 

 very base or at any distance upwards to within about 1 5 /x of their 

 apices. Thus conjugation has been observed to occur over four-fifths 

 of the length of the conidia measured from their bases. Conjugation 

 has never been seen to occur at the very apices of two conidia, and 

 this may be due to the fact that the apices taper to a point and, in 

 general, are far apart relatively to other portions of the two shafts. 



Rawitscher 3 and Boss 4 have shown that, during conjugation of 



1 Vide supra, pp. 30-33. 



2 A. F. Blakeslee, " Sexual Reproduction in the Mucorineae," Proc. Amer. 

 Acad, of Science, Vol. XL, 1904, p. 274. 



3 F. Rawitscher, " Zur Sexualitat der Brandpilze : Tilletia tritici,' 1 '' Ber. d. 

 Deutsch. Bot. Oes., Bd. XXXII, 1914, p. 310. 



4 G. Boss, " Beitrage zur Zytologie der Ustilagineen," Planta, Bd. Ill, 1927, 

 pp. 619-322. 



