ON THE CAUSES OF THE ZONING OF BROWN SEAWEEDS ON THE SEASHORE. BY SARAH M. BAKER. (University College, London).

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1 196 Sarah M. Baker. time, this does not explain tbe maintenance of an enormous maximum through the cold period (4" 1"C) when in both the pre. ceding years the greatest maximum was attained at a temperattire of approximately 9"C. Botanical Laboratory, Birmingham University. ON THE CAUSES OF THE ZONING OF BROWN SEAWEEDS ON THE SEASHORE. BY SARAH M. BAKER (University College, London). [TEXT-FIGS ]. THE conditions which determine the zonal growth of Algae between the tidal limits appear likely to be very simple, and thus afford a very promising field for experimental study. The present paper is a contribution in this direction. The actual zonal distribution of Algse on the shore has been very thoroughly studied by Borgesen,^ and others. But in order to have a clear understanding of the conditions under which tbe seaweeds were growing at the particular locality chosen for the present work, a set of measurements was taken to find their vertical distribution in relation to the tides. The experiments were carried out on the eastern side of White Cliff Bay, near Bembridge, Isle of Wight, where there is a continuous stretch of gently inclined limestone and marl rocks upon which the Alg^ grow profusely, for a distance of about two miles along the coast. The level of high water at the spring tide was found by marking from a boat the highest point to which the tide rose on a given rock. This point was used as the standard point of reference. At low tide an observer was stationed at this rock, while two others went down and found the seaweeds. The vertical distance below 1 Borgesen. Om Alge-vegetationen ved Faeroernes Kyster. Gyldendalske Boghandel, Nordisk Forlag. Kjobenhavn et Kristiania, Also Idem. The Algal Vegetation of the Faroese coasts, with remarks on the phyto-geography. Botany of the Faroes based on Danish investigations. Copenhagen, 1905.

2 The Catises of the Zoning of Brown Seaweeds. 197 high water was found hy sighting across from the reference point to the horizon, and adjusting a slider on a graduated vertical scale into line with this. Sighting to the horizon involves an error of not more than half-an-inch (1.2 cm.) in the more distant readings taken {i.e. about 100 yards or metres). By this method the vertical height of any rock could be measured to within an inch or so. Several readings were taken at different points on the rocks for each species of seaweed. The zoning seemed to be similar at all points on the rocks measured. The zones merged into one another, but were on the whole very well defined. The readings were taken as far as possible on gently inclined rocks, and no account was taken of seaweeds growing in rock pools, or even in hollows which might form temporary rock pools, because the seaweeds in the pools were often entirely different from those on the rocks near Thus in a rock pool high up on the shore, in the zone of Fucus ceranoides, there were flourishing specimens of Fucns serratus and also of Halldrys slllquosa, which are usually found below the low water of the neap tides. The following Table shows the extreme readings, that is those taken on the extreme edges of each zone, and also mean readings taken from the middle of the zones, where each species was at its thickest. VERTICAL DISTANCES BELOW H. W. S. T. Spring Tide... High Water Feet Metres Low Water = 4-0 Neap Tide... High Water... 3 = Low Water... 9 = 2-7 Species of Seaweed. Upper Limit. Lower Limit. Mean Reading. Fucus ceranoides Ascophyllum nodosum Fucus veslculosns - Fucus serratus ft ins. mtrs. 9 = = 0-7 9= = 1-7 ft ins. mtrs. 0 = 0-9 3= = = 3-7 ft. 2 3 to 5 9 ins. mtrs. 6 = 0-8 6= ft. to 1-2 6= = 2-7 Halidrys slllquosa 11 6 = 3-5 Lamlnarias = 3-9 These readings have been plotted graphically on the diagram (Text-fig. 27). It appears that the time during which the seaweeds

3 Sarah M. Baker. IE in in _L Fig. 27. Diagram representing vertical distribution of Algje. The horizontal scale is measured at right angles to the line of the shore. The upper side of the diagrammatie section shows the lowest point to which each speeies descends, while the lower side shows the highest point to which each species ascends.

4 The Causes of the Zoning of Brown Seaweeds, i 99 were covered by the sea was the primary factor in determining the zoning. The rock-pool vegetation confirms this. The seaweeds growing high on the shore were never, or very rarely, found in pools, whereas those which were only uncovered at the lowest tides were found in a great majority of rock-pools. ZONING Fig. 28. In water 11 hours; dry 1 hour. x. Fucns scryatus grew rapidly at first, but flagged later. Fiiciis vfsiciuosiis grew rapidly; no specimen died. Ascophylliiin iwdosuin grew well; no specimen died. Fiiciis ccraitoidcs gvew slowly ; no specimen died. There are three phases of the plant's life history which may be influenced by drying, vis : 1. Germination of the zygote. 2. Vegetative growth. 3. Reproduction and dispersal of gametes or zygotes. The first and last of these phases offer some difficulties to experi-

5 ioo Sarah M. Baker. mental study, owing to the small size of the gametes. To see whether it would be possible to grow tbe seaweeds under different conditions with regard to dessication from those to which they were accustomed, a considerable number of very small nlants were collected off the rocks, each species being taken from the very centre ZQNINC or SEAWEEDS YOUN& GHOVJN 6 ih W.ATER 6 H«e, SURVIVOR OF 6? A6C0PHYU-UM NODOSUM Fig. 29. In water 6 hours ; dry 6 hours, x. Fucus semttiis ; most of the specimens died in about two weeus. Fucus vcsiciilosus grew quickly, but became somewhat shrivelled. Ascophyllum nodosum g<'e\v well ; no specimen died. Fucus ayanoides grew best under these conditions. of its zone ; these were divided as equally as possible between three jars. The sea-water in the jars was changed once every twelve hours; but during tbe course of the twelve hours one jar (A) was left dry for one hour and under water for eleven hours; the second (B) was dry for six hours and under water for six hours ; the third (C) was dry for eleven bours and under water for one hour. By

6 The Causes of the Zoning of Brown Seaweeds. 201 this means a rougb imitation of the periodic exposure by tbe tides was obtained. Tbe plants were treated in tbis way for twentyfour days, and even in tbis comparatively sbort period tbey sbowed market! differences according to the conditions. Text-fig? sbow tbe effects on the growth of tbe different species. YOUNG SEMWCC i>,s c IN FOCUS ALL E-!SrD /f 4 ; \ f f / / FUCtl6 F KA Fig. 30. In water 1 hour; dry 11 hours. X I. Fiicus serratiis died in a few days. Fuciis vesiciilnstis grew very little, and became shrivelled at the base. AscopliyUiim nodosum : growth very stunted. Fuciis (cnvwides; grew slowly ; no specimen died. Tbe specimens of Fucus serratus died in two or tbree days in jar C. In jar B most of the plants flagged in ten to fifteen days, but one was was still alive at the end of tbe experiment. In jar A they grew very quickly, and after tbe first six or seven days were far ahead of the other seaweeds in tbe jar; but after this they flagged and one or two of the specimens finally died.

7 2O2 The Causes of the Zoning of Brown Seaweeds. The specimens of Fucits vesiculosus grew rapidly in jar A, and none of the plants died. In jar B this seaweed grevr quickly, but some of the specimens became ratber shrivelled towards the end of the time. In jar C only two specimens just survived, grew very little and shrivelled at the base. Ascophyllum nodosum grew well in jars A and B, and one specimen survived in C, though much stunted. Fucus ceranoides was very slow growing in all the jars, but it grew best in B, where there was exposure for half the period. None of the specimens died. From the way in which the seaweeds grew, one would draw the following conclusions : 1. That the species of seaweeds growing high up on the shore have a power of resisting dessication, which is not possessed by those growing lower down, and that this power decreases regularly in those species growing towards the lower levels. 2. That the seaweeds which can best resist dessication grow most slowly, and those that grow most quickly are the least tolerant of dessication. The species which resist dessication most mtist have most protection from loss of water, and hence will absorb water most slowly; and since these plants get tbeir nourishment from general absorption of water by the thallus, tbe best protected plants must grow most slowly. In the case of Fucus ceranoides there seems to be a definite physiological adaptation against dessication, for it grows best when it is dry for a large proportion of the time. It is probable that, growing on the rocks, the seavi'eeds would never become so dry as in the glass jars, which were left exposed to all the weather conditions; but the results obtained give an indication of the reasons for the zoning observed. In the lower zones the primary factor is probably rate of growth; the quickest growing forms would supersede the others. In the upper zones the determining factor is tolerance of dessication. It is easy to see that these factors would cause fairly sharp zoning of the forms witb these varying properties. There may be also other factors depending on the reproductive systems of the adult plants on the dispersal of the gametes, and on the power of germination of the zygotes under different conditions. I have to thank my brothers for kind assistance in carrying out this work.

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