^4 



PHYCOMYCETES 



o p 



called conidiophore, of Phytophthora (Fig. 17), the main branches showing 

 a series of contractions which mark the bases of successive lateral branches. 



The sporangia fall off when ripe, and the contents of each then divide up 



to form about eight motile /oospores 

 (Fig. 17 C and D). These are as a 

 rule only liberated when plenty of 

 moisture is present, swimming about 

 in drops ot (.lew or rain on the plant, 

 finally coming to rest and starting the 

 disease afresh. 



Sexual reproduction is also met with 

 Vt A '$/} in the family. The sexual organs are 



oogonia (female) and antheridia 

 (male). The former are large, terminal, 

 spherical cells at the ends of some of the 

 hyphae; the latter are smaller and more 

 or less club-shaped, growing close to the 

 oogonia, but usually arising from separate 

 hyphae (Fig. 1 8). When both are mature 

 the antheridium puts out a very slender 

 fertilising tube which penetrates 

 the wall of the oogonium ; through 

 this some of the contents of the antheri- 

 dium pass into and fertilise the single 

 egg-cell. The latter then surrounds 

 Fig. 16. Phytophthora infestans. A. Lower itself with a thick cell-wall and becomes 



an oospore \ this remains dormant 

 until it is eventually set free by the 

 decay of the surrounding tissues of the 

 diseased host-plant. Only the oogonia 

 and oospores of Ph. infestans are known 

 at present, the antheridia still remaining 

 to be discovered. (See Kew Bulletin 1913, p. 192.) 



Closely allied to the potato disease is the "False Mildew" of the vine, 

 caused by the fungus Plamopara viticolii. Here the conidiophores arise in 

 tufts from the stomata of the leaf, and each is very much branched, the 

 ultimate branches being very small and short and arranged in groups of three 

 (see Fig. 19). In other respects it closely resembles Phytophthora. 



The second family of the Algal Fungi are the Zygomycetes, the only 

 well-known member of which is the common white mould, Minor. This is 

 described in so many botanical text-books that the reader is referred elsewhere 

 for an account of its life history. 



Related apparently to both these families is the " locust-fungus," Empusa 

 Grylli. This is entirely parasitic on the bodies of locusts, quickly killing the 



* Continuation from page 20. 



1 6. Phytophthora infestans. A '. Lower 

 surface of potato leaf, showing the 

 sporangiophores protruding from the 

 stomata. 90/1. B. Mature sporan- 

 gium. C. The same, its contents 

 aggregated. D. Motile spore (zoo- 

 spore). B-D 540/1. 



(After H. Schenck) 



