FIG. 12
A simple form of the plankton collecting tow-net.
A tow-net can be bought from the laboratory of the Marine Biological Association at Plymouth (address: The Laboratory, Citadel Hill, Plymouth) or it can be home-made. It consists essentially of three parts: in front is a hoop made either of light galvanised iron or strong cane and provided with three bridles of cord which will come together at a small ring or shackle for attaching to the towing rope; next comes the actual net, a conical bag made of a fabric which will act as a fine sieve; lastly at the end of the net is a small collecting jar, either a glass honey-jar or one made of zinc or copper with a slight lip. Such a simple tow-net is shown in Fig. 12. For ordinary collecting purposes a hoop of 18 inches diameter will be quite sufficient. The net is best made of the silk ‘bolting cloth’ used by millers for sieving flour, but a good quality muslin will do if this cannot be obtained; with a mouth of 18 inches it should be almost five feet in length. If it is to be homemade great care should be taken in cutting out the material in order to ensure that a perfect cone is formed; if lop-sided it will not fish properly. It is a good plan to pin together a paper model to serve as a pattern; this will also enable one to see how best to use the material with as little waste as possible. Round its wide mouth a canvas or calico band is sewn for attachment to the hoop; it may either be provided with a series of eyes for lashing it on or it may be folded over the hoop and sewn to. enclose it, leaving gaps where the towing bridles are secured. At the hind end is sewn another canvas band to form a small cylinder, say 2½ inches in diameter, which will slip closely over the mouth of the collecting jar and be firmly held in position by a tightly tied tape. It is well to be provided with two such tow-nets; one made of the very finest material for the collection of the small plants—the finest bolting-cloth has 200 threads to the linear inch—and one of coarser material, having about 60 threads to the inch, for the capture of the somewhat larger animals. The coarser net lets most of the plants go through its mesh but filters a very much larger quantity of water more quickly and so captures the larger more active animals which are only rarely taken in the finer net.
To collect the phytoplankton the fine net should be towed just a little way below the surface. A weight, say a 71b. lead, is slung at the end of the rope and the net attached a little way above it. The essence of successful tow-netting is to tow very slowly, never at more than 1½ knots. If it is towed faster the water will not be filtered quick enough; the net will just push a mass of water in front of it which will prevent any more water entering it. A ten minutes’ tow may give quite a large enough sample. Most of the plankton will have passed down into the jar at the end as it is towed; a number of specimens, however, may still be sticking to the inside of the net as it is taken from the water, so that it should be carefully washed down from the outside with water from a bucket, to flush them into the jar.
Our sample will contain a vast number of both plants and animals. In this chapter we will concern ourselves only with the former, which are so small that they must be looked for with the compound microscope. After bringing our sample home and letting it stand for a little we should take only a few drops at a time with a fine pipette from near the bottom and place them on a slide under a coverslip; now we shall hunt with the low-power lens and then turn on the high-power to examine each new specimen we find. We shall not, of course, expect to find examples of all the different kinds in one sample but there may well be representatives of several of the more important groups. The most prominent members of the phytoplankton are the diatoms. They are unicellular algae differing from all other algae in having a cell wall which forms a siliceous external skeleton enclosing the cell like a glass box. The pigment bodies, or chloroplasts, which enable the plant to make use of the energy of sunlight are not the usual bright green of chlorophyll but a brown or brownish-green pigment closely allied to it. The siliceous skeleton is in two parts which fit together like the top and bottom of a pill-box; indeed some of the diatoms are just like a pill-box in form, but many others are drawn out into all manner of fantastic shapes. When first we see a sample of plankton rich in diatoms under the high power of the microscope it is like looking at a group of crystal caskets filled with jewels as the strands of sparkling protoplasm and groups of amber chloroplasts catch the light. Every plant or animal cell consists of a mass of protoplasm with a more or less central body, the nucleus, which appears to govern its life; it is characteristic of the diatoms that, in addition, the protoplasm usually has large cavities in it containing clear fluid. The nucleus is usually central and surrounded by a mass of protoplasm; radiating from this and forming an irregular network are protoplasmic strands stretching across the cavities like the spokes of a wheel to join up with a layer of protoplasm which lines the inner surface of the boxlike covering. More rarely, in some forms, the nucleus may be in the layer of protoplasm at the side. The pigment granules usually lie more or less regularly spaced against the cell-wall, where they are exposed to the light; if, however, the light is too intense, they come close together either down the strands to the centre or to some other part of the cell where they can partly screen one another from the harmful effects of the rays.
The top, bottom and sides of the glass-like box are not made of just plain sheets of silica; their surfaces are sculptured with all manner of striations, pits and perforations forming intricate patterns peculiar to the different species. This detail of design has always made the diatoms favourite specimens with microscopists, not only on account of their beauty, but because they are such excellent objects with which to test and display the quality of their instruments in the higher ranges of magnification. They are now being put under the electron microscope which can give a micrograph with a magnification of up to 100,000 times; this has at once revealed an arrangement of structure far more elaborate than that seen with the highest powers available in the optical systems (Hendy, Cushing and Ripley, 1954). Instead of there being just one system of pits or perforations in their walls, some forms are shown to have smaller and yet smaller ones, secondary and tertiary systems, on inner layers of silica; in others the wall is more like a basket of spiral threads intricately woven together. Some of the perforations measured were less than a ten-thousandth of a millimetre in diameter and the surrounding walls were of equal thickness. These delicate lattice systems have at least two important qualities for floating plants: they give strength with lightness and at the same time provide a framework for presenting a greatly increased surface area of protoplasm to the surrounding water.
FIG. 13
Diagrams showing the division of a simple pillbox-like type of diatom. a, a sketch of the cell before division has begun, b to d sections through the diatom after cell division to show stages in the formation of the new skeletal cell walls. Note that the upper cell in d is smaller than the original cell b.
Diatoms normally reproduce by simply dividing in two. The nucleus divides first and then the protoplasm becomes separated into two masses, each containing a nucleus, one at either end of the box; each mass of protoplasm now forms, between it and the other mass, a new valve as the halves of the pill-box are called. These new valves each fit closely their own part of the old box; we have in fact two pillboxes now instead of one, as is shown above in Fig. 13. They may separate entirely, or in some species they may remain attached to form long chains. It will be realised that in this process of repeated division by forming new half-boxes within the old, the average size of the diatoms so produced will tend to get smaller and smaller; at each division, as shown in the drawing, one of the new boxes will be the same size as the old one but the other must be smaller. Thus we find a considerable range in the size of diatoms of the same species, but there must be a limit to this reduction. After a certain number of such divisions there is formed what is called an auxospore, by which the original size is recovered; throwing off the old valves the cell becomes a bladder-like mass of protoplasm within which new valves are formed two or three times the