July 1, 1891.] 



KNOWLEDGE. 



137 



era n 111 u i->i 

 wall 



ture, easily find their way to the young tubers beneath. 

 The cuticle of these is so delicate that it affords no barrier 

 to the entrance of the gonidial-tube. 



According to De Bary, the processes described are the 

 only means by which the fungus can reproduce itself, and 

 the reader will doubtless be of opinion that such means are 

 ample, A few years ago, however, Mr. Wortbington 

 Smith described the formation of sexual organs, and his 

 observations were corroborated by other workers in this 

 department of Botany. In leaves of the potato, towards 

 the late autumn, and in diseased tubers, the products of 

 the sexual organs are, as a rule, plentiful. A small branch 

 of the mycelium, in one of the intercellular spaces, swells up 

 in a globular fashion, and is cut off by a septum (division) 

 from the rest of the hypha, to which, however, it is st 11 

 completely adherent. On the same, or on a neighbouring 

 hypha, a tubular protuberance arises which applies itself 

 to the swelling just mentioned. Its contents are also 

 divided from those of the rest of the hypha by a mem- , 

 branous partition. The first formed body is called the 

 (iiiriii)iii(iii, or egg producing apparatus, the latter is the 

 litilli'wiiUuiii (oiJa, like) for a reason presently obvious (see 



Fig. in.). Both are 

 densely filled with 

 protoplasm. That of 

 the oogoninm divides 

 into two parts, an 

 inner more granular 

 portion surrounded by 

 a clear, delicate, cell 

 wall, and called the 

 ovum, or egg, and an 

 outer less granular 

 case the periphifim 

 {tdpi, around, and 

 irAair/xa, a thing 

 moulded). The pro- 

 toplasm of the 

 pollenodium exhibits 

 difl'erentiation of a 

 like kind, although not so markedly. The ovum before 

 fertilization has received the name oosphere. Where the 

 pollenodium touches the oogonium, a small tube is sent 

 out, which penetrates that organ and so reaches the 

 anxjilicic. Then the central part of the polleuodium's 

 protoplasm, the i/oikijiIhsiii {yBwduj, to produce), is poured 

 into the oospore, which is now called a fertilized egg, or 

 tioaiiiirc. The connection between the oogonium and the 

 parent mycelium now ceases, and changes immediately 

 take place in the former. A wall of cellulose is formed 

 round the oospore, the periplasm contracts, and the wall 

 of the oogonium remains as a covering until germination 

 takes place. The oospore is a little spore, and the sac 

 containing it, previously the female reproductive oogonium, 

 is now a sporangium (Fig. III.). 



The spore does not germinate at once. Ten months 

 elapse before this can take place. The product of germin- 

 ation is one or more germ-tubes, which behave exactly 

 like those of the gonidia ; or the spore, like the gonidium, 

 breaks up into a number of moving bodies, each furnished 

 with two cilia, and so comparable to the zoogonidia. 

 These arc called zoospores, because their parent was a 

 spore. 



If we adopt Mr. Worthington Smith's view of the case, 

 tlie spores are the resting stage of the fungus. It is 

 almost impossible in any case to imagine that the mycelium 

 is perennial in the tubers, that is, that it can live in these 

 more than oiu> year, when we know its putrefactive action 

 on their cells. Yet it has been held by some that the 



Fig. III. — The reproductive organs 

 of Phijtophthora infestans (?). rt'unil 

 It, (liagraiiiniatie, e {ad iirtt.). 



mycelium is perennial. At present, however, the matter 

 may be considered suh-jmlitr. 



The oogonia and pollenodia described by ^Ir. Worthington 

 Smith have been held to belong to an allied fungus, a 

 species of pythium. Dr. Plowright's remarks, in the 

 lecture already referred to, are interesting in this con- 

 nection : " All of us who have studied the potato disease 

 have hunted for these resting spores. A few years ago, 

 Mr. W. G. Smith thought he had found them, and most 

 of us thought so too, but it was- subsequently shown by 

 the late Professor De 

 ^nidio ^_ ^ Bary, that spores closely 



resembling Smith's 

 resting spores, very com- 

 monly occurred in potato 

 plants kept in damp 

 situations, which be- 

 longed to a species of 

 pythium. Since this time 

 we have been unable to 

 meet with a sexual spore, 

 which upon germination 

 is capable of giving rise 

 to the phytophthora, and 

 although the probability 

 is that such a spore does exist, yet there the matter rests." 

 The temperature of the air has a great influence on the 

 development of this fungus. Above 77° F. the mycelium 

 is killed, and below 40° F. the production of gonidia ceases. 

 About 70° F. may be reckoned as the most productive 

 temperature. In its native place, as we formerly remarked, 

 the potato was undoubtedly attacked by this parasite, 

 although for well-nigh 300 years it had not made its 

 appearance in I^urope. It has been affii-med that the high 

 temperature of the regions surrounding its native habitat, 

 localised that of the phytophthora, and also that in passing 

 through the torrid zone any infected tubers were sterilized, 

 that is, were completely ridded of their dread enemy. 

 Nowadays, steamers have replaced sailing vessels, making 

 the passage through the torrid zone of exceedingly short 

 duration, so that the fungus is not killed in the transit of 

 the potato from Chili to Europe. Doubtless also, the same 

 authority remarks, the guano trade has done much to 

 further the spread of this pest, whose life-history can be 

 seen at a glance by reference to Figure IV. 



Fig. IV. — Diagrammatic repre- 

 sentatives of life-historv of 

 Phytophihora infeafnuti (tlic 

 potato-disease fungus). 



THE FACE OF THE SKY FOR JULY. 



By Herbert S.ydler, F.E.A.S. 



THE solar surface is still active. Until about the 20th 

 of the month there is no real night in the British 

 Islands. The following are conveniently observable 

 times of the minima of some Algol type variables 

 (<;/'. " Face of the Sky " for .Tune) : Algol. — 

 .Julv Kith, Oh. 33m. a.m. ; Julv 19th, !)h. 22m. p.m. 

 S Librie.— .July 4th, lib. 8m. i'.m^ ; .July 11th, lOh. 43m. 

 P.M. ; July 18th, lOh. 17m. p.m. ; July 25th, Oh. 51m. p.m. 

 U Coroni¥. — Julv 2Stb, lib. lOm. p.m. U Ophiuchi. — 

 .Jidy 5th, lib. Om. p.m.: July 10th, llh. 47m. p.m.; 

 .July leth, Oh. 32ui. a.m. : July 2(Uh, lOh. 12m. p.m. ; 

 July 31st, lOh. 57m. p.m. Y Cygni, July 2nd, Oh. 12m. 

 P.M. ; July 5th, Oh. 7m. p.m. ; July 8th, Oh. 2m. p..m. ; 

 July 11th, 8li. 57m. p.m.; July 14th, 8h. 51m. p.m. 

 Variable of short period, not of Algol type, -q AquUw. — 

 July 5th, 111. .\.M. Maximum of S Ursie Majorii (<;/'. 

 Mr. Peek's paper in Knowled(;e for March, 1890) on 

 July 24th. 



^lercury is not favourably situated for observation in 



