I 



TEANSACTIONS OF SECTION D. 777 



cycads. The specimen whicli supplies the most important evidence in the ques- 

 tion as to relationship with living plants is one from the collection of the late 

 Mr. Binney. In many of the vascular bundles of this Oldham example the phloem 

 is well preserved, and a longitudinal section clearly demonstrates the position of 

 the protoxylem. In examples previously described and figured the position of the 

 protoxyleui has been inferred from the appearance, in transverse sections, of the 

 xylem tracheids ; in the longitudinal section of the Oldham Myeloxylon the small 

 xylem tracheids next to the phloem are seen to be spirally thickened, and are 

 without doubt protoxylem. 



Another example of Myeloxylon described was found in a limestone band of 

 Millstone Grit age : this furnishes facts of some interest in connection with the 

 question of botanical affinity, and as regards fossilisation. Hitherto English 

 specimens of Myeloxylon have only been found in the Coal-measures. 



6. Observations on tJie Stmchire of Cystopns Candidas. 

 By Harold W. T. Wagek. 



In this paper a short account is given of the structure of the vegetative and 

 reproductive organs of Ct/stopus candtdus, a fungus parasitic on cruciferous plants, 

 and especially abundant on the common shepherd's-purse — Capsella bursa-pastoris 

 — on which it forms white shining patches, often of considerable extent on the 

 stem, leaves, and fruits. 



The mycelium consists of non-septate hypha3 which ramify in all directions 

 between the cells of the host plant, and often produce extensive hypertrophy of 

 the organ attacked. Here and there small spherical haustoria are developed on 

 the hyphse, and these penetrate directly into the cell cavities. 



Both hypliie and haustoria contain a granular protoplasm, in which numerous 

 nuclei can be seen. The structure of these nuclei can be made out by careful 

 staining and examination under a sufficiently high power, as was shown by Fisch, 

 with whose observations mine in the main agree. Each nucleus is vesicular, and 

 contains a large nucleolus which stains deeply and is surrounded by a less deeply 

 stained substance, in which some indications of a granular or thread-like structure 

 may at times be observed. 



Tue reproductive organs are of two kinds, asexual and sexual. The asexual 

 organs are formed just beneath the epidermis of the host plant : they are club- 

 shaped branches of the mycelium, from which zoosporaugia are cut off by con- 

 striction and formation of a double cell wall at the apex. Tliese branches contain 

 granular protoplasm derived fi-om the mycelium, and numerous vacuoles. A few 

 nuclei are found scattered in the protoplasm, the number behig variable, from five 

 to twelve. The structure of the nucleus is similar to that in the mycelium, but is 

 more easily made evident. 



In the formation of a sporangium a part of the protoplasm accumulates at the 

 apex, together with a few nuclei, four or five, or perhaps more ; this is then sepa- 

 rated from the club-shaped branch by constriction of the cell membrane. A 

 double cell wall is then formed which completely cuts oil' the sporangium from 

 the branch on which it is formed. The sporangium is thus, from the beginning, 

 a multinucleated cell. Each nucleus at a later stage becomes the nucleus of a 

 zoospore without undergoing further division. 



The nuclei found in the club-shaped branches are probably derived directly 

 from the mycelium, no division stages having been observed in the sporangiophore 

 itself. It is possible, however, that owing to the smallness of the nuclei the 

 division stages are masked. 



The sexual organs, antheridia and oogonia, also contain numerous nuclei. 

 According to Fisch the oogonium contains from ten to twenty nuclei, the anthe- 

 ridium thi-ee or four. I find more than this in many cases. In the formation of 

 the oosphere the protoplasm separates into periplasm and gonoplasm, a cell wall 

 appears round the gonoplasm ; a large number of nuclei remain in the periplasm, 

 but the gonoplasm does not appear to possess nuclei at this stage ; at a later stage. 



