Algae, Fungi, and Lichens. 275 



with which each is provided, and, seeking the egg, one 

 of them fuses with it and accomplishes its fertilization. 

 However, proterandry may occur, so that the egg is often 

 not fertilized by sperms from the neighboring antheridium. 

 The egg while still contained within its cell then produces 

 a thick wall about itself, and passes through a resting 

 period before germinating. Vaucheria is a step in advance 

 of Spirogyra in the evolution of sexuality, for its repro- 

 ductive elements are quite sharply differentiated into egg 

 and sperm. 



168. Fucus. The large brown marine Alga known as 

 wrack-weed or bladder-wrack (Fucus vesiculosus) grows 

 anchored to the rocks between high and low tide levels. 

 Its branches are somewhat flattened, and possess air 

 bladders which buoy it up in the water, and thus enable 

 it to expose a broad surface to the light. It is multicellu- 

 lar, and its tissues are somewhat differentiated, the central 

 part consisting of slender cells with large spaces between 

 them containing mucilage, and the outer tissues being 

 made up of smaller rounded cells, the peripheral layer of 

 which has the nature of an epidermis. Chloroplasts reside 

 in the outer tissues, but their green color is modified by a 

 brown coloring matter associated with the chlorophyll. 

 Notwithstanding this, the chloroplasts of Fucus are active 

 in photosynthesis and sustain the same relation to the 

 sunlight in the manufacture of food materials as do the 

 chloroplasts of green plants. 



169. Reproduction of Fucus. Reproduction is effected 

 sexually by the production of eggs and sperms, borne in 

 minute cavities of the plant body, known as conceptacles 

 (Fig. 139). In the species vesiculosus the sperms and 

 eggs are borne in different plants, but in other species 

 they both occur in the same conceptacle. The eggs, 



