changes indicating its 'pseudomorphism' into a parasite-spore. Besides these, other parasitic swarmers are found which become somewhat amoeboid (schwach amöboid werden') and bore into the vegetative hyphae; these are larger than the former and possess a pale nucleus. They may enter the antheridium and pass down the fertilizing tubes into the oogonium, and according to Zopf form zoocysts therein, but do not prey on the spores. The smaller amoebae, and to some extent the larger, he regards as identical with Pringsheim's spermamoebae. Now while it is obvious that in many respects Zopf's larger amoebae correspond with my species, he refers them in his monograph to the genus Vampyrellidium1, and species V. vorax Z., which differs in that the zoospores do not pass through a mastigopod stage, and, from his figures (Fig. 37), in the characters of the zoocyst which has granular peripheral plasma, with a central nucleus surrounded by hyaloplasma; and finally in preying on green Algae as well as on Saprolegnieae. This species I have not found; but I cannot help thinking that he must have overlooked the differences between it and mine, which is unmistakably that figured by Pringsheim and recognised by Lindstedt, and which I have never found absent from old cultures. While in his paper he says they do not attack the oospores, in his monograph he says they wander 'in die vegetativen Schläuche sowohl als in die Oogonien und Antheridien hinein, nähren sich vom Inhalt dieser Organe, und bilden in ihnen schliesslich auch Dauersporen.' I have repeatedly seen my organisms destroy the young oospores before encysting in the oogonium, so that the same oogonium may contain zoocysts of the parasite and oospores of the Fungus; and they sometimes even attack the ripe oospores despite the protection of the thick cell-wall. I think it very possible that Zopf may have confused two species here. His phrase 'Schwärmer, welche schwach amöboid werden' would seem to imply that they were previously flagellate, and hence could not belong to Vampyrellidium.

1 Die Pilzthiere oder Schleimpilze, 101.

Pringsheim in his reply denies that the large amoebae can have anything to do with his spermamoebae. He admits that amoebae may be seen in empty organs which are no longer closed, and that Chytrideae and their allies can, of course, in the swarming stage penetrate healthy normal organs, but observes, justly, that these have nothing to do with Zopf's amoebae. He denies that the amoebae so often seen crawling over the hyphae ever penetrate them, and refers Zopf's observations to a confusion with the cellulincorpuscles, which by the way I may note he was the first to correctly describe and name in a full study in a later paper1. His words are worth quoting: 'Es sind frei im Zellumen der Saprolegnieen-Schläuche und in den Oogonien niedergeschlagene und im Alter geschichtete Körner aus einer Art Pilzcellulose, oder eine verwandte Modification derselben. ... Es sind diese körnige Niederschläge des Zellinhalts, welche derselbe [Zopf] für zu Ruhe gekommene Amöben und Spermamöben angesehen hat.' On this identification I may note that it is obviously wrong, as the cellulin-granules always disappear in the formation of spores, and it is only in hyphae emptied by the action of parasites that I have ever found them persist. Pringsheim's own old figure of Leptomitus with a zoocyst of the Monadine side by side with a laminated cellulin-body, is ample proof of the inaccuracy of the last conclusion. I have frequently seen an amoeboid of the form of an acute isosceles triangle, with the angles rounded and the base anterior in locomotion (Amoeba limax' form), crawling about my cultures; but this is quite distinct from the amoeboids of any monadine 2.

It is strange to find that Pringsheim should have fallen into this confusion, and that neither he nor possibly Zopf appears to have seen the mastigopod stage. I can only account for it by the hour at which swarming takes place, an hour not likely

1 Berichte d. Deutsch. Bot. Gesellschaft, 1883.

2

Pringsheim also answered Zopf in a paper in his own Jahrbücher (vol. xiv. 1884, p. 111, Nachträgliche Bemerkungen z. d. Befruchtungsact von Achlya), but he gives no further account of the larger amoebae which alone concern us here, nor does he even refer to his previous reply to Zopf in the Centralblatt.

to be chosen by a man who has leisure for original research in the daytime.

It is obvious that, as my organism is probably identical with Zopf's larger amoebae, while his smaller ones are equivalent to Pringsheim's spermamoebae, the foregoing study does not directly solve the question as to the fertilization or apogamy of the Saprolegnieae. Yet in so far as it proves that Pringsheim has confused a parasitic with a normal structure in the one case, it raises a presumption that he may have made the same mistake in the other; and I have a strong body of evidence from a totally different source tending to prove that his theory is erroneous. But this belongs of right to my nearly completed study of the protoplasmic structures of the Saprolegnicae and will be fully discussed therein.

EXPLANATION OF FIGURES IN PLATE XXII. B.

Illustrating Prof. Hartog's paper on a Monadine parasitic on Saprolegnieae.

Figs. 1, 2 (× 750 E). Two young swarmers as drawn in the infested hyphae which contains also mature zoocysts; nucleus seen.

Fig. 3 (× 750 E). Young amoeboids in the living hyphae of Achyla. Two contractile vacuoles shown, one in the moment of reconstitution after systole by the confluence of three minute ones. n=nucleus; co=contractile vacuoles.

Figs. 4, 5, 6 (× 750 E). Older amoeboids from within a hypha; 5 and 6 are successive stages of the same; a few bright (faecal ?) granules present.

Figs. 7, 8 (× 750 E). A migrating amoeboid inserting a pseudopodium into a living hypha; two successive stages.

Figs. 9-13 (2 mm. apochr., 12 Comp. Oc. x 1000; owing to their being balsam specimens this is equivalent to a magnification of 666 x the living organism). 9, young zoocyst with faecal mass (e) and nucleus; 10, same, with seven daughter nuclei visible (eight in all); 11, 12, further stages of spore-formation, the sporeorigins somewhat unduly separated by shrinkage; 13, the spores completely separated.

Fig. 14 (same magnification). Balsam specimen at commencement of zoocystformation, with flagellum protruded.

Fig. 15 (× 750 E). Same stage living; some of the faecal granules still in the protoplasm.

Fig. 16 (× 750 E). Live zoocyst, divided partially into spore-origins by radiating vacuoles.

Fig. 17 (× 350 E). Portion of infested hypha of Achyla with zoocysts and cellulin-bodies (c).

Besides the magnifications, the objective of Zeiss under which the drawing was made is given.

On antithetic as distinct from homologous Alternation of Generations in Plants.

BY

F. O. BOWER, D.Sc.,

Regius Professor of Botany in the University of Glasgow.

HERE are few subjects within the scope of Biology

THER

which have given rise to so much divergence of opinion, and at the same time so great obscurity of conception as that of 'alternation of generations.' For more than half a century the fact that within the narrow limit of the ontogenetic cycle, like does not always directly produce like, has been known and discussed, the discussion most frequently taking the form of mere comparison of the successive phases of various organisms, with but slight reference, if any, to the external circumstances under which the organisms grow, or to their relationships by descent. At the present time, knowing as we do how profoundly the environment affects the conformation of the organism, it is imperative that in the discussion of the phenomena of alternation such considerations should be constantly kept in mind, and especially the differences of external conditions of the organisms in which alternation is seen.

Before the days of the theory of evolution, when the idea. of uniformity of type in organic creation held stronger sway over the minds of biologists than now, it was natural that the attempt should be made, by coercing facts into correspondence, to draw comparisons where they are not warranted: even among those who accept evolutionary views, the tendency [Annals of Botany, Vol. IV. No. XV, August 1890.]