44 BOTANY. 



branching of the plant is brought about. Scattered over the 

 surface are little circular pits through whose openings pro- 

 trude bunches of fine hairs. When wet the plant is flexible 

 and leathery, but it may become quite dry and hard without 

 suffering, as may be seen when the plants are exposed to the 

 sun at low tide. 



The air bladders are placed in pairs, for the most part, and 

 buoy up the plant, bringing it up to the surface when covered 

 with water. 



The interior of the plant is very soft and gelatinous, while 

 the outer part forms a sort of tough rind of much firmer con- 

 sistence. The ends of some of the branches (Fig. 26, A, x) 

 are usually much swollen, and the surface covered with little 

 elevations from which may often be seen protruding clusters 

 of hairs like those arising from the other parts of the plant. 

 A section through one of these enlarged ends shows that each 

 elevation corresponds to a cavity situated below it. On some 

 of the plants these cavities are filled with an orange-yellow 

 mass ; in others there are a number of roundish olive-brown 

 bodies large enough to be easily seen. The yellow masses are 

 masses of antheridia ; the round bodies, the oogonia. 



If the plants are gathered while wet, and packed so as to 

 prevent evaporation of the water, they will keep perfectly for 

 several days, and may readily be shipped for long distances. 

 If they are to be studied away from the seashore, sections for 

 microscopic examination should be mounted in salt water 

 (about 3 parts in weight of common salt to 100 of water). If 

 fresh material is not to be had, dried specimens or alcoholic 

 material will answer pretty well. 



To study the minute structure of the plant, make a thin cross- section, 

 and mount in salt water. The inner part or pith is composed of loosely 

 arranged, elongated cells, placed end to end, and forming an irregular net- 

 work, the large spaces between filled with the mucilaginous substance 

 derived from the altered outer walls of these cells. This mucilage is hard 

 when dry, but swells up enormously in water, especially fresh water. 



