600 MR JOHN RATTRAY ON ECTOCARPUS. 



absolute size than the other cells of the thallus. This stimulation is no doubt 

 caused by the waste products evolved by the parasite as well as by the very 

 presence of the latter inside the host cell. Moreover, in both cases, there is 

 the same final result, namely, complete devitalisation of the organic substance, 

 and the ultimate death of the host. 



All the figs, on Plate CXLVIII. have been drawn by the aid of Abbe's 

 camera : fig. 7, under Leitz's microscope Oc. 1, Ob. No. 10 Hartnack (water 

 immersion) ; the other figures under Zeiss's microscope Oc. 3, Ob. CC. 



EXPLANATION OF PLATES CXLVIL, CXLVIII.. 



Plate CXLVIL represents a specimen of Ectocarpus siliculosus richly covered with the parasitic Rhizo- 

 pJn/tliiyn Dicksonii. The latter occurs in various stages of development on the ultimate and penultimate 

 ramuli of the host which hears none of the ordinary fructification. 



On Plate CXLVIII. various conditions of the parasite may he ohserved. 



Eig. 1 shows a host cell enlarged, rounded, and containing a Rhizophydiuni of considerable size, and 

 bounded by a faint though distinct cell wall. 



Eig. 2, a, represents the earliest stage of the parasite that has been observed, while the adjoining cell 

 (fig. 2, 7;), contains a large reniform Rhizophydium, which, however, has not caused the usual bulging 

 of the host cell wall. 



Ei< r . 3 shows a condition in which three adjoining host cells have been attacked. 



Eig. 4. Here the host cell is slightly swollen, and the parasite, instead of being perfectly round, has 

 expanded somewhat in a lateral direction, and shows slight involutions on two opposite sides. 



Eig. 5 represents a perfectly elliptical Rhizophydium lying in a somewhat swollen host cell. 



Eig. 6 shows a parasite which possesses a remarkable indentation on one of its margins. The host 

 cell is very much enlarged and rounded. 



Fig. 7 is a very much enlarged representation of a parasite which has just ruptured. The broken 

 margins of the host cell wall are well seen, as also the creases of the cell wall of the parasite. 



Figs 8a, 8b, and 8c show the positions of the points of rupture of the cell wall of the parasite, with 

 reference to the direction of the axis of the filaments of the host plant. 



Eig. 9 shows a remarkable involution of the cell wall of the host plant, rupture having taken place 

 at the diametrically opposite point. 



Fig. 10 shows a peculiar reticulation of the spores of the parasite, which occurs just before rupture 

 of its cell wall. 



Fig. 11, a, represents the angulated condition in which the spores exist at and before the period of 

 rupture. This is the result of mutual pressure caused by the continued growth. In fig. 11, b, a very 

 much enlarged cell is seen, which is also obviously affected by the presence of a parasite. 



In fig. 1 2 the angulation of the parasitic spores, the creasing of the cell wall, and the positions of 

 rupture of the parasite, may be studied. 



