A well-known ally of the tangles is the “merkal,” also called “honey-ware.” You can tell it by the prominent midrib and the broad, thin wing on each side, running all its length. This is one of the edible seaweeds. Do you see this bright red palmate plant growing under the shelter of the tangles? It is the common dulse (_Rhodymenia palmata_), which may often be seen for sale on the streets of our cities. Examine it well and taste it, and you will be able to recognize it in future, however much it may vary in form or colour. But do not eat too much of it, for it is said to be somewhat indigestible. Another edible seaweed which has been widely used as an invalid food may be found in the lower part of the “ebb,” often under the shelter of larger plants. This is the Irish moss or carrageen (_Chondrus crispus_). It is fleshy and pink in colour. A jelly is made from it which is considered a great delicacy. The purple laver (_Porphyra_) is perhaps the most valuable of the seaweeds as a food, and is said to sell at a high price in Yokohama. In form it resembles the sea-lettuce. Many other marine algæ have been used as food, and none of them are poisonous. In North Ronaldsay the sheep seem to esteem them highly as food. The most important use of seaweed is to serve as food for various kinds of molluscs, crustaceans, and fishes. The “plankton” of the sea-surface—minute one-celled algæ—are very important in this way. What grass is to the land animals, the marine algæ are to the living creatures of the sea. When driven ashore by the waves, or when cut down by the once familiar “hook,” the larger seaweeds are much used as manure for field crops. They thus repay the debt they owe for any portion of their food that may have come originally from the dry land. Before returning from our walk let us haul down this small boat from its “noust” and take a bird’s-eye view of the seaweeds in their natural habitat. Through the clear water beneath us we can see the strange shapes of the submerged vegetation, dense and tangled, with here and there a lazy sea-urchin on the broad red-ware, and the sillocks actively swimming around. But our oars are entangled in the “drew” (_Chorda filum_), so full of annoyance and even of danger to the swimmer. Look at one of those long threads. It is covered with hairs; it tapers towards both ends, and its fructification extends along its whole surface. In structure it is a hollow tube divided into many chambers. What a variety of colours and shades we see as we look down on this wonderful submarine scenery! We notice that near high-water mark green is the predominant colour, and that the lower belt is mostly brown, while here at low-water mark and beyond it, as well as under the shelter of the sea-wracks and tangles, shades of red prevail. Beyond the depth of thirty or forty fathoms seaweeds are extremely rare, owing to the want of light at the sea-bottom: seaweeds, like other plants, cannot take in their food in darkness.