grazing would be “inducible” chemical defense, which does not occur in most brown algae (Luder and Clayton, 2004). Synthesis of phlorotannins may be induced by the growth regulator jasmonic acid as it is in higher plants (Arnold et al., 2001).

Eggs of the Phaeophyceae contain phenolic vesicles just under the plasma membrane that are discharged outside the cells by exocytosis after fertilization. It has been postulated that the discharge of these phenolic vesicles has a toxic effect on spermatozoids and acts as a polyspermy block before the primary wall is secreted (Clayton and Ashburner, 1994). These peripheral phenolic vesicles are distinguishable from physodes, which also contain phenolic compounds, but which are significantly larger and tend to be localized around the egg nucleus.

Life history

The unilocular sporangium (Fig. 21.7(b))is generally considered to be the site of meiosis, the haploid zoospores that are released forming the gametophyte generation. The gametophyte then produces the gametes, which fuse to form the

zygote (Bell, 1997). Although meiotic divisions have been considered the rule in the unilocular sporangium, a disturbingly large number of investigations have not found this to be the case. In these investigations there is a “direct” type of life history, with no meiosis or fusion occurring. More research is needed in this area to clarify the situation. In the phaeophycean life cycle, a plethysmothallus is a filamentous stage (or one composed of compacted filaments) that can multiply itself by spores (usually zoospores from plurilocular sporangia) (Papenfuss, 1951).

The thallus of many Phaeophyceae is relatively large and complex with a number of different types of growth that include: (1) diffuse, with most of the cells of the plant capable of cell division (Ectocarpus, Fig. 21.17 and Petalonia, Fig. 21.20);

(2) apical, with a single cell at the apex giving rise to the cells beneath (Dictyota, Fig. 21.11 and Sphacelaria, Fig. 21.13); (3) trichothallic, where a cell divides to form a hair above and a thallus below (Cutleria, Fig. 21.14 and Dermarestia, Fig. 21.15); (4) promeristem, with a non-dividing apical cell controlling a large number of smaller meristematic, dividing promeristematic cells beneath it (Fucus, Fig. 21.41); (5) intercalary, with

Fig. 21.8 The chemical structure of some brown algal pheromones with the names of the algae that secrete the pheromones. (Modified from Pohnert and Boland, 2002.)

a zone of meristematic cells forming tissue above and below the meristem (Laminaria, Figs. 21.24, 21.23); (6) meristoderm, with a layer of usually peripheral cells dividing periclinally (parallel to the surface of the thallus) to form a tissue below the meristoderm (usually cortex) and occasionally anticlinally (perpendicular to the surface of the thallus) to add more cells to the meristoderm (Fucus, Fig. 21.41).

Investigations on the sexual hormones of the brown algae in the 1970s and 1980s by Müller (Fig. 21.9) and his associates represent a major advancement in the knowledge of the brown algae (Müller, 1982; Maier and Müller, 1986). A sexual hormone (sirenine, pheromone) is a diffusible substance that coordinates cellular activities during sexual reproduction. Two types of biological effects mediated by sexual hormones occur in the brown algae: (1) the explosive discharge of spermatozoids from antheridia, and (2) the attraction of male gametes by female gametes or eggs. All sexual hormones in the brown algae are unsaturated hydrocarbons (have at least one double or triple bond) (Fig. 21.8) (Müller et al., 1982). With the exception of the Fucus sperm attractant, all of the sexual hormones in the brown algae are C8 to C11 olefins (unsaturated

open-chain hydrocarbons containing at least one double bond), most of them incorporating a fiveor seven-membered ring structure. All of the attractions in the brown algae identified so far are highly volatile and hydrophobic. Very small amounts of the pheromones are released, only 0.6 fmol per cell per hour from Ectocarpus siliculosus gametes. However, the non-polar nature of the pheromones contrasts strongly with the highly polar nature of water, making the pheromones easily recognizable to the responding cells. Perception of the pheromone starts with the simple partition from water of the non-polar pheromone into the lipid plasma membrane of the gamete that contains the macromolecular receptors (Pohnert and Boland, 2002). The substances have a very strong tendency to leave the aqueous solution and escape into the air. This characteristic helps to avoid the buildup of chronic concentrations which would decrease the efficiency of the gradients around the female cells. The attractants probably do not function more than 0.5 mm away from the female cells, and thus attraction is clearly a short-distance phenomenon (Müller, 1982). The sexual hormones include ectocarpene from Ectocarpus (Müller et al., 1971), desmarestene from Demarestia aculeata

(Müller et al., 1982), lamoxirene from Laminaria (Müller et al., 1985a,b), multifidene from Cutleria multifida (Jaenicke et al., 1974), dictyopterene C from Dictyota dichotoma (Müller et al., 1981), and