[ 



EUPHYCOPHYTA 209 



or intercalary positions on the ramuli, and, as in some of the other 

 genera, only one ovum develops in each megasporangiimi, the 

 remaining seven nuclei degenerating. In the sporeHng the main 

 shoot is very short and soon stops growth, and a new apical cell 

 arises near its base (Fig. 117 d). This new apical cell gives rise to 

 the adult axis which is therefore a branch of the sporeling. The 

 later laterals develop monopodially and in succession. The first 

 two shoots arise opposite each other but the remainder have a 

 divergence of 2/5. The genus is principally confined to the warm 

 sub-tropical and temperate waters of the globe. 



*Sargassaceae: Sargassum {sargasso ^ Spanish for seaweed). Fig. 

 118 

 In the simplest forms branching is distichous but in the great 

 majority it is radial with a divergence of 2/5. Like Cystoseira the 

 main axis is usually very short and the thaUus is primarily com- 

 posed of richly branched, long laterals. Basically the leafy short 

 laterals borne on the long laterals are built up as follows: the 

 primary branch is a sterile phylloclade which bears cryptostomata, 

 whilst the secondary branch is also sterile and is commonly reduced 

 to an air bladder. In the simpler forms the subsequent branches 

 are fertile and finger-like in appearance. In more advanced forms 

 there may be further sterile phylloclades which may subtend axil- 

 lary branch systems. In branching the main apical cell (a), (Fig. 

 117 e) cuts off a secondary initial (aj) and this latter, as soon as it 

 has emerged from the apical pit, cuts off another initial (ag) on the 

 distal side. This last initial divides more rapidly than the other and 

 grows out into the subtending leaf, which is therefore a lateral that 

 has pushed aside the parent axis. Later the parent initial (a^ pro- 

 duces further initials on a 2/5 divergence. The plants are attached 

 by means of a more or less irregular, warty, solid, parenchymatous 

 base or else numerous stolon-hke structures grow out from the 

 main axis and anchor the plant. The genus, which is principally 

 confined to tropical waters, is a very large one with about 150 

 species, some being dioecious whilst others are monoecious. Mor- 

 phologically the structure of the species is similar to that for Fucus 

 though there is Uttle or no medullary material, and when present it 

 is not gelatinized. In the ripe oogonium (megasporangium) only 

 one ovum will normally reach maturity, though occasionally 



