598 MR JOHN RATTRAY ON ECTOCARPUS. 



light was so adjusted, by means of the mirror, that about two-thirds of the 

 field was dark, while the remaining third was illuminated with diffuse daylight, 

 and it was observed that the true reproductive swarmspores of this species had 

 similar negative heliotropic tendencies, as they congregated on the dark part of 

 the field." In a genus closely allied to Ectocarpus, viz., Sphacelavia plumigera, 

 I am informed by Mr Traill that Air E. M. Holmes, F.L.S., has recorded 

 similar heliophobic movements of the swarmspores. It may be added, by 

 way of contrast, that, according to BoRZi,t the zoogonidia of Ulva are posi- 

 tively heliotropic, while the conjugated zoospores have opposite or heliophobic 

 tendencies. 



The significance of such movements, in relation to the light, may be of im- 

 portance as affording an early indication of the habit of the plants. Thus many 

 marine algre avoid exposed sunny places, and delight in dark rocky crannies, 

 where the rocks, from their softer nature, have become eroded by wave action, 

 aided by the denudatory effects of the impact of hail, snow, rain, &c. That 

 such seaweeds tend to accelerate marine erosion cannot be doubted, when it is 

 remembered that their rhizoids, in many cases, become entangled among minute 

 particles of shale or other soft rock, and that the plants possess a certain 

 buoyant power which tends to lift the fragments, to which these rhizoids are 

 fixed, from their place. This buoyant power, moreover, is often largely in- 

 creased by the passage of waves which play against the thallus, and so increase 

 its pull upon the rock to which it is fixed. 



Although swarmspores, that are thus heliophobic, tend in the first instance 

 to settle down upon shaded water- worn spots, it does not follow that round 

 mammillated or exposed rocks are never covered by algae, whose spores exhibit 

 such a tendency. It is to be borne in mind, on the contrary, that a heliophobic 

 spore may often find enough of shade among the rhizoids of other pre-existing 

 weeds, while its rhizoids will in turn form a suitable nidus for another similarly 

 disposed spore, so that finally a round exposed protuberance may be entirely 

 covered with alga) whose spores are negatively heliotropic. 



In connection with what has been stated above, with regard to the escape of 

 the multilocular swarmspores of Ectocarpus siliculosus, it is of interest to record 

 that the period during which spores may escape in that species can no longer be 

 limited to the summer season, as has hitherto been done in the basin of the 

 Firth of Forth, but must be extended to December. It is, however, to be borne 

 in mind, that the unusual mildness of the season, combined with the sheltered 

 locality in which the specimens were found, may have largely contributed to 

 this result. It is also worthy of remark, that a rise of temperature has a direct 

 influence in hastening the escape of the swarmspores, inasmuch as specimens 



* In tins case direct rays of light were carefully excluded from the stage of the microscope, 

 t Borzi, Studi abjolocjici, 117 pp. (9 pi.). Messina, 1883. 



