594 MR JOHN RATTRAY ON ECTOCARPUS. 



fact that it has entered a cell which is not very rich in organic contents, in 

 which it is enabled to develop without producing so great internal pressure 

 as must necessarily be exerted when the latter is tensely filled with fluid cell 

 sap and other protoplasmic substances, or derivatives from these. 



A still more remarkable appearance is that represented in fig. 6. Here the 

 host cell has become distinctly globular, and contains a well-defined parasitic 

 Rhizophydium, around which a more or less hyaline area is found, in which the 

 chlorophyll granules show manifest pathological changes. Instead of being 

 absolutely round, this parasite has a well-defined and moderately deep indenta- 

 tion on one margin, around which its cell wall passes without interruption. It 

 is not easy to give a satisfactory reason for the existence of this fold, although 

 two possible explanations may be offered, namely, it may be due to the exist- 

 ence of a greater internal pressure existing in the host cell at this point, a slight 

 divergence from the perfect equilibrium being sufficient to cause a sinuosity in 

 the thin and pliant cell membrane, or if this parasite should ultimately be found 

 to possess this indentation in its earlier stages, it may point to a specific 

 distinction between it and the perfectly ovoid or globular type. As only a 

 single instance of this peculiar phenomenon was observed, it is not unlikely 

 that the former explanation is the more trustworthy. 



At the moment of rupture the cells containing the mature Rhizophydia 

 present in most cases a distinctly ovoid appearance. That the internal tension 

 in these cells is at this time great is proved at once by their external form, by 

 the fact that they bulge into adjoining cells of the host plant which are not 

 so affected, and by the existence of creases which may not uncommonly be 

 observed just after the rupture (fig. 12). It is also to be noted, that as soon as 

 relief from this tension is obtained, the adjoining cells of the thallus of the host 

 plant at once become convex on the side of the ruptured cell in virtue of their 

 own elasticity, and so tend to cause a more violent rupture, and to facilitate the 

 dissemination of the swarmspores of the parasite. 



In cases where three adjoining cells contain parasites (fig. 3, a, b, c), the 

 middle cell, which is unable to indent the walls of the adjoining tense cells, 

 expands usually in a somewhat lateral direction, and always towards that side 

 on which the resistance is least when this tendency is first manifested. 



After rupture takes place the appearances presented by the affected cell 

 areas are sometimes of a very fantastic kind. The cell wall of the parasite, as 

 soon as the wall of the host cell opens, suddenly swells up, owing to the pressure 

 exerted by the contents against its inner surface, in many cases to twice its 

 former dimensions (figs. 7, 10, 11, &c). At first the parasitic cell membrane is 

 entire (fig. 10), but it soon opens to the exterior, sometimes by a single pore 

 (fig. 9), which may be directed either towards the base of the filament, or 

 towards its apex, or at right angles to its longitudinal axis, sometimes by two 



