MR JOHN" RATTRAY ON ECTOCARPUS. 593 



The genus Rhizophydium was established by Schenk for Chytridiaceous 

 parasites, whose sjjores escape by one or more apertures. The globular and 

 unicellular Rhizophydium Dicksonii has hitherto only been noted on Ectocarpus 

 granulosus gathered at Howth, near Dublin, during the winters of 1876 and 1877. 

 I, however, gathered specimens of Ectocarpus siliculosus which were affected 

 with this parasite on December 4, 1884, about 10.30 a.m. The host plant was 

 found growing epiphytically on a fragment of Polysiphonia fibrittosa, which was 

 obtained about the level of f -tide in the old Granton Quarry. A small part of 

 the shale on which the Polysiphonia grew was removed at the same time, so that 

 the epiphyte and its parasite were disturbed as little as possible. The tempera- 

 ture of the water at the time of removal was 41 0, 2 Fahr. Some of the speci- 

 mens of Ectocarpus thus obtained were placed in a small glass vessel containing 

 sea water, and kept for some days in the microscopic room of the " Ark," the 

 floating laboratory connected with this station. This material was repeatedly 

 examined to determine when rupture of the cells would be likely to take place' 

 and this occurred about 1.30 p.m. on December 7, when specimens were at once 

 mounted in weak acetic acid. The temperature of the water in the vessel in 

 which rupture occurred was 51 °8 Fahr. 



The parasitic Rhizophydial cells were found to be widely disposed over the 

 penultimate, but sometimes occurred on the ultimate ramuli of their host plant 

 (PI. CXLVIL). In the former they were not confined to any definite region, but 

 were found scattered at irregular intervals along their entire length. They usually 

 occurred singly, but at other limes were found in pairs (PI. CXLVIII. fig. 8), while 

 in a few cases three contiguous host cells were attacked (PL CXLVIII. fig. 3). 



The parasitic cells enter the cells of the host plant at a very early stage of their 

 existence, and gradually develop at the expense of the highly organised proto- 

 plasmic contents of the latter. In PI. CXLVIII. fig. 2, two adjoining host cells 

 have been penetrated by the Rhizophydial cells, which have arrived at different 

 stages of development; PI. CXLVIII. fig. 2, a, represents the earliest stage which 

 I have observed. The parasite presents the appearance of a nucleus, and is 

 surrounded by a thin but definite cell wall. In figs. 1, 4, and 5 of the same Plate 

 somewhat more advanced stages of development are represented, and while in 

 fio-s. 1 and 5 the Rhizophydium has assumed an almost perfectly ellipsoidal form, 

 in fio\ 4 it exhibits a tendency to expand laterally, being even slightly involuted 

 at one end, on account of the greater resistance to which it is exposed in this 

 region. At this stage the presence of the parasite is, in ordinary circumstances, 

 revealed by the slightly globular form assumed at a very early period by the 

 host cell containing it. Sometimes, however, as in the case represented in 

 fio\ 2, b, the parasitic cell may grow to a moderate size without producing 

 the ovoid bulging of its containing host cell, and without itself retaining the 

 normal globular or ellipsoidal outline. This is probably attributable to the 



