OOMYCETES 



11 



into the hyphal wall. The middle layer elongates, thus becoming 

 narrower and is finally dissolved, serving as a disjunctor. Meanwhile 

 new nuclei have entered the sporiferous hypha and are used in the forma- 

 tion of another conidium. Apparently, nuclear divisions do not occur 

 in conidiophores and conidia. 



Peronosporeae. — Here only one conidium arises at each hyphal tip; 

 between it and the hypha, there is formed a single short sterigma of a 

 water-soluble substance which facilitates the abscission (Rostowzew, 

 1903). The disadvantage of this single abscission is compensated in 

 various forms in a definite manner; thus the conidiophores of Basidio- 

 phora are still unbranched, as in Albugo; at their tips they are slightly 



Fig. 46. — Albugo Candida. 1 to 4. Formation of conidia. Albugo Portulacae. 5. Catenu- 

 late conidia. (1 to 4 X 900; after Bary and Rosen.) 



clavate and closely covered with numerous small processes, each of 

 which cuts off a conidium (Fig. 47, a) ; in this manner, there arises on a 

 single conidiophore a large number of spores. Also in Bremia the tips 

 of the branches are swollen and covered with short processes. In the 

 other genera there appears a tendency to repeated branching. In some 

 species of Plasmopara (Fig. 48, A) and in part in Sclerospora, there is 

 present a typical monopodial principal axis from which radiate shorter, 

 often trichotomous branches. In most species of Peronospora the 

 conidiophores form regular trees with a high trunk and a definitely 

 differentiated crown whose branches are forked dichotomously up to 

 twelve times. How effective this forking of the conidiophores is may 

 be seen from a calculation of Weston, where for S. philippinensis the 



