110 



THE AGRICULTUKAL NEWS. 



April 2, 1910. 



FUNGUS NOTES. 



THE CHIEF 



OF FUNGI. 



GROUPS 



Part III. 

 In the last article, a short account was given of the four 

 main groups into which the fungi are divided. It now 

 remains to discuss these groups and some of their more impor- 

 tant subdivisions in somewhat greater detail. This will be 

 the object of the remaining articles of the series. 



The PHYCOMvrEi es. The group of fungi which contains 

 the most primitive forms is that referred to in the last article 

 as t)ie Ph_ycomycetes. It is subdivided into two main 

 branches : the Oomycetes and the Zygomycetes. The distinc- 

 tive characters of these are as follows. In the ( )omycetes, 

 asexual reproduction is by means of sporangia producing free- 

 swimming zoospores, as described in the case of the Chytridi- 

 neae*, or the contents of a sporangium may grow out at once 

 and form a germ tube. Sexual reproduction is by means of 

 two specialized organs usually formed on short lateral branch- 

 es of the hyphae. The female organ is known as the 

 ongoniuiii, or egg- forming organ: the male as the (inthirifliuin. 

 (Fig. 17.) 



In the Zygo- 

 mycetes, asexual 

 reproduction is 

 b y m e a n s of 

 small non-motile 

 spores produced, 

 usually in large 

 numbers, in a 

 .sporangium. The 

 other form of re- 

 production gene- 

 rally takes place (a) Antheridium. 

 by means of two 



similar hyphae, which are not sexually differentiated as far as 

 can be seen outwardly. The tips of these two hyphae fuse, 

 (become intimately joined together, the walls between them 

 being absorbed) and a .spore is produced at the point of fusion. 

 Such a very simple form of the fertilizing process is known as 

 Conjugation. (Fig. 19 ) 



The Oomycetes. — As an example of the 

 life-history of one of the Oomycetes, that of 

 riii/tiijihtltora omnii'oni may be described. 

 This fungus causes the black rot disease of 

 cacao pods. The mycelium grows in the 

 tissues of the host, and produces short 

 external hyphae at right angles to the sur- 

 face: each of these is usually once or twice- 

 branched, and the branches bear terminal 

 large, pear-shaped, conidia. The conidia, 

 when ripe, are distributed by the wind, and 

 when they alight on the surface of another 

 host plant under favourable conditions 

 germinate, and either form numerous 

 zoospores, each possessing a single cilium, 

 or produce several germ tubes which penetrate into the 

 host plant and form a mycelium directly. This shows 

 that the conidium is really a transformed zoosporan- 

 gium, or organ for producing zoospores. The zoospores, 

 after swinmiing for some time in any moisture that there 

 may be on the host plant, come to rest and also produce 

 one or several germ tubes, as described for the conidia, from 

 which tha mycelium is formed. Occasionally, a conidium 



1 a 



Fig. 17. (1) Coxidi.\, (2) Sexu.\l PiKI'RO- 



nucTioN OF I'liytophthora omttivora. 

 (o) Oogonium. 



Fii;. 18. SpoRAJf- 

 GiUM OF Mucor 

 mucedo, with 



CoLUMELL.^. 



*This word was 

 tyjiogiaphical urmr. 



iiisspelt in till 



last article, owini; to 



may gi\e rise to only one short hypha, with a small lateral 

 branch, which then bears two terminal secondary conidia 

 similar to the original one. 



Sexual reproduction takes place inside the tissues of the 

 liost. A short branch grows out from one of the hyphae of 

 the mycelium and becomes very much swollen at the end. The 

 swollen end then becomes separated from the hypha which 

 carries it, by a cross wall. This swollen end is the oor/onium, 

 and contains the female portion of the sexual spore. (Fig 17, o.) 

 While this is being formed, another lateral hypha arises near 

 the first, and its tip is also cut off by a wall, though it does not 

 swell up as the oogonium does. This is the male organ, or 

 antheridium. (Fig. 17, a.) The tip of the antheridium fuses with 

 the side of the oogonium, and its contents pass into the oogo- 

 nium and fertilize the egg. After fertilization, the egg cell 

 surrounds itself \^ith a strong thick wall and remains lying 

 inside the old wall of the oogonium. These sexual spores 

 are onlj' liberated by the decay of the tissues of the host 

 plant; this leaves them lying free on the surface of the soil. 

 They are able to germinate and reinfect another host plant as 

 much as four years after their original formation. On germina- 

 tion, they form one or more short hyphae, which almost imme- 

 diately produce conidia at their tips. This, then, is the life- 

 hi.story of Phytophthora ontnirora, and that of most of the 

 Oomycetes is very similar. The group includes many well- 

 known parasites, as for example, Pi/thiinn de Bari/anum, 

 which causes the damping oft of many seedlings, the grape 

 mildew (Plasmopora vi(irola) and many others. It iiresents, 

 moreover, a series of gradational forms, from the most ele- 

 mentary, as instanced in the Chytridineae, to much more 

 elaborate ones, such as Phytaphthora, and other members 

 of the cohort Peronosporineae. 



The ZYGOMVCETES. — ■ 



As an illustration of 

 the life history of one 

 of the Zygomycetes, 

 that ofthe Muoorsmay 

 be described. These 

 fungi are mostly sa- 

 prophytes, living on 

 many different sub- 

 stances. The mycelium 

 closely resembles that 

 of the Oomycetes, but 

 the reproductive ar- 

 rangements are some- 

 what ditterent. In the 

 asexual stage, erect 

 Fic. 19. Staces IX THE Cax.TUNCTioN hyphae are produced 

 OF Mucor stolonijh: at right angles to the 



underlying tissue (sub- 

 stratum). These become swollen at the end, and the swollen 

 portion is cut off by a wall, forming the spore-producing 

 organ, or sporangium. The wall then swells upward into the 

 hollow of the sporangium, and give rise to a central portion, 

 called the i-olumella. (Fig. IS.) The contents ofthe sporan- 

 gium divide up to produce numerous minute spherical spores, 

 which are liberated by the bursting of the sporangium wall, 

 and are then distributed by the wind. 



Tlie other kind of spore is formed as follows: The tips of 

 two neighbouring hyphae become somewhat swollen, and each 

 is cut off' by a cro.ss wall. They eventually touch one another 

 and fuse at the point of contact, and their protoplasmic 

 contents unite; in this way a simple form of fertilization 

 is brought about. (Fig. 19.) The spore, consisting of the 

 two swollen ends of the hyphae, becomes surrounded with 

 a thick wall, and is isolated by the decay of the original 



/ 



