MR JOHN EATTEAY ON ECTOCAEPTJS. 597 



This is explicable on the supposition that the parts of the cell wall adjoining 

 other cells continue to receive relays of food molecules from the latter, when 

 they are no longer able to procure them from the protoplasmic material which 

 occupies the lumen of the cell which they themselves bound, so that devitalising 

 changes are retarded most, just in the immediate vicinity of these adjoining 

 cells, but less and less as we advance from that point, and least of all opposite 

 the middle of the cell lumen where rupture is effected. 



Among the extrinsic causes that induce rupture, there maybe noted (1) the 

 pressure exerted by the contents of adjoining cells which are not similarly 

 affected with the parasite. The operation of this cause becomes plain when it 

 is remembered that the bulging of the affected cell causes the cell walls, bound- 

 ing adjoining cells, to become concave on the side next itself. This sets up an 

 elastic pressure in the contents of these adjoining cells, which in turn acts on 

 cells next them, and so on. When rupture of the ovoid host cell takes place, 

 the hitherto concave walls of the neighbouring cells become convex, and so 

 bulge into the ruptured cell, thereby causing a wider dissemination of its con- 

 tents than would otherwise be the case. 



(2) The impact of ripples or foreign bodies against the swollen host cells, 

 the former of which especially occurs when the tide just reaches the level at 

 which the plant grows, have obviously a similar tendency. 



Negative heliotropism of the swarm spores of Rhizopliydium. — After a large 

 number of the swarmspores of Rhizopliydium had escaped into the watch-glass 

 in which the host plants lay, the glass was surrounded, to the level of its brim, 

 with black paper, so that no light could penetrate into it from beneath. The 

 surface was then covered as closely as possible to the level of the water with a 

 similar piece of black paper. Before being covered on the surface, the water 

 in the glass was stirred gently with a needle, so that the swarmspores were 

 equally diffused through it. The cover when adjusted was allowed to remain 

 for several hours, and it was so arranged as to conceal the surface of fully 

 three-fourths of the glass. On re-examination of the water by removing two 

 samples of it simultaneously with two very small clipping tubes, and placing them 

 under microscopes, it was found that while the water which had been exposed 

 to the daylight contained a few swarmspores, the greater number of the spores, 

 which were still in active motion, had congregated on the dark edge of the glass. 

 Their general tendency must, accordingly, be regarded as negatively helio- 

 tropic. 



In connection with this subject, it may at this point be noted, that in the 

 same gathering of Ectocarpus siliculosus, many specimens were obtained which 

 bore the ordinary multilocular fruit figured by Harvey.* The biciliate swarm- 

 spores that escaped were observed for some hours under the microscope. The 



* Harvey, Phyc. Brit. 

 VOL. XXXII. PART III. 5 G 



